Nitro 4.88 gear heat

[linuxgod]

We went camping this weekend, and the new Nitro 4.88 gears pulled the trailer great. My 6000# trailer felt like it was maybe 4500#. The one thing I noticed was that the engine and transmission temp were higher than normal when monitoring in OBD Fusion. Historically on the highway (at least around here in flatlandia) if the torque converter is locked I'll see engine oil, coolant, transmission pan, and torque converter temps all hover between 196F and 200F, almost in sync (typically 196-198F, but if it's really hot out or we get up a few thousand feet in elevation like western Nebraska I'll see it hit 200F or maybe 1-2F higher). I used to tow in 4th gear all day long.

With the new gears I tried towing in both 4th and 5th. In both scenarios I can lock the torque converter up, at least on Illinois highways. But in both scenarios as I got above 55-60 mph the transmission, torque converter, and engine oil temps all slowly creep up. Coolant temp stayed stable at 199F. 4th or 5th gear didn't matter, so this wasn't a matter of "can't get A/T fluid through the system fast enough. Ambient temps were 75-77F so not that hot out. Since I was driving I couldn't take precise notes, but it looked pretty much like this, give or take maybe a degree):

Speed MPH	Coolant Temp	Engine Oil Temp	Transmission Pan Temp	Torque Converter Temp
55-60	199	199	199	199
65	199	203	203	203
70	199	207	207	207
75	199	210	210	210

Temps color coded for easy viewing. I checked and the engine oil and coolant levels looked good.

Given it's both transmission temps and engine oil temps that are running about 10F hotter than normal, and the only significant change is the new gears, I'm thinking the hotter temps are gear related. I have two possible theories, but wanted to know pool the group to see their experiences and thoughts and if this seems normal to those who have re-geared or if this is as concerning to others as it is to me:

1. Not enough airflow across front diff to keep it cool. My temps were pretty stable before, and I've heard shorter gears can run hotter since they're spinning faster, so perhaps the gears are generating more heat. Airflow hasn't changed (though my front bumper+winch does change the airflow from stock), but if I was pretty close to the limit that the truck could cool before (no way to know this) perhaps now I'm over that limit? I'm thinking of dropping the skids to test this theory during my next camping trip on June 10. Just a bit of hassle to do this in the garage
2. Incorrect diff fluid type/weight. I went with synthetic Redline 75W-140 in an attempt to manage lubrication under heavier loads since about half my driving is typically with the trailer in tow. Is the excess heat a good thing (meaning the fluid is extracting heat from the gears and transferring into the aluminum housing) or is the fluid too heavy (viscous) even under load and perhaps I should be running the Toyota 75W-90 fluid (or something else)? I could just swap it, but I only have about a month to make adjustments and only really one more camping/towing trip planned before I have a long road trip, so if 75W-90 will make it worse I don't want to waste time (and $) testing that
3. Other possibilities I'm not thinking of?

Tagging @Taco2Cruiser on this question in particular since I know Rob has added a temp gauge to at least his rear diff, and I'm hoping to his front as well. That said I'm hoping @grinchy or others who have done Nitro 4.88s might have other real-world experience with the above,

FWIW I'm not too concerned about the extra heat for occasional local trips, but I am a bit concerned that when I get out west and the ambient temps are 95-100F and the air is a bit thinner at a few thousand feet of elevation where *normally* my temps might get to 201-202F suddenly I'm not going to be able to control the heat without slowing waaay down.

 

[TLC2013]

We went camping this weekend, and the new Nitro 4.88 gears pulled the trailer great. My 6000# trailer felt like it was maybe 4500#. The one thing I noticed was that the engine and transmission temp were higher than normal when monitoring in OBD Fusion. Historically on the highway (at least around here in flatlandia) if the torque converter is locked I'll see engine oil, coolant, transmission pan, and torque converter temps all hover between 196F and 200F, almost in sync (typically 196-198F, but if it's really hot out or we get up a few thousand feet in elevation like western Nebraska I'll see it hit 200F or maybe 1-2F higher). I used to tow in 4th gear all day long.

With the new gears I tried towing in both 4th and 5th. In both scenarios I can lock the torque converter up, at least on Illinois highways. But in both scenarios as I got above 55-60 mph the transmission, torque converter, and engine oil temps all slowly creep up. Coolant temp stayed stable at 199F. 4th or 5th gear didn't matter, so this wasn't a matter of "can't get A/T fluid through the system fast enough. Ambient temps were 75-77F so not that hot out. Since I was driving I couldn't take precise notes, but it looked pretty much like this, give or take maybe a degree):

Speed MPH	Coolant Temp	Engine Oil Temp	Transmission Pan Temp	Torque Converter Temp
55-60	199	199	199	199
65	199	203	203	203
70	199	207	207	207
75	199	210	210	210

Temps color coded for easy viewing. I checked and the engine oil and coolant levels looked good.

Given it's both transmission temps and engine oil temps that are running about 10F hotter than normal, and the only significant change is the new gears, I'm thinking the hotter temps are gear related. I have two possible theories, but wanted to know pool the group to see their experiences and thoughts and if this seems normal to those who have re-geared or if this is as concerning to others as it is to me:

1. Not enough airflow across front diff to keep it cool. My temps were pretty stable before, and I've heard shorter gears can run hotter since they're spinning faster, so perhaps the gears are generating more heat. Airflow hasn't changed (though my front bumper+winch does change the airflow from stock), but if I was pretty close to the limit that the truck could cool before (no way to know this) perhaps now I'm over that limit? I'm thinking of dropping the skids to test this theory during my next camping trip on June 10. Just a bit of hassle to do this in the garage
2. Incorrect diff fluid type/weight. I went with synthetic Redline 75W-140 in an attempt to manage lubrication under heavier loads since about half my driving is typically with the trailer in tow. Is the excess heat a good thing (meaning the fluid is extracting heat from the gears and transferring into the aluminum housing) or is the fluid too heavy (viscous) even under load and perhaps I should be running the Toyota 75W-90 fluid (or something else)? I could just swap it, but I only have about a month to make adjustments and only really one more camping/towing trip planned before I have a long road trip, so if 75W-90 will make it worse I don't want to waste time (and $) testing that
3. Other possibilities I'm not thinking of?

Tagging @Taco2Cruiser on this question in particular since I know Rob has added a temp gauge to at least his rear diff, and I'm hoping to his front as well. That said I'm hoping @grinchy or others who have done Nitro 4.88s might have other real-world experience with the above,

FWIW I'm not too concerned about the extra heat for occasional local trips, but I am a bit concerned that when I get out west and the ambient temps are 95-100F and the air is a bit thinner at a few thousand feet of elevation where *normally* my temps might get to 201-202F suddenly I'm not going to be able to control the heat without slowing waaay down.

Thanks for sharing. Good stuff here. Going to be following this closely to see what our Mud gurus have to contribute.

 

[grinchy]

I haven't done any temp information and don't have a scangauge installed at the moment. I did get an obdfusion capable OBD port dongle for christmas, but haven't done anything with it yet. So that's a long way of saying I can't really further the discussion from a data point of view. I also haven't done a second fluid change on the gears yet (10k on them), but I can say the housing doesn't glow red after a long drive.

I find it non intuitive that the internal engine oil reading matches to the Transmission fluid temperatures. Also that the engine coolant isn't influenced by the warmer transmission temperature or warmer engine oil temp. If there is a meta system here it seems that there would be more interaction or influence on the common factor (coolant), and that the engine oil and transmission fluid would track separately. So perhaps double check the sensor mapping?

Are the observed temps outside the 'safe' range?

 

[Whiskerz]

Oil is going to be alright at 210. you can pretty safely run up to 240 or even a hair more. You may want to shorten up your change intervals. Engine temps are good obviously. Just on principle you may want to add an external cooler. As for the front diff in your #1. Nothing you have measures either diff temperature stock. That is in the land of class 8 trucks and off highway equipment

 

[linuxgod]

I haven't done any temp information and don't have a scangauge installed at the moment. I did get an obdfusion capable OBD port dongle for christmas, but haven't done anything with it yet. So that's a long way of saying I can't really further the discussion from a data point of view. I also haven't done a second fluid change on the gears yet (10k on them), but I can say the housing doesn't glow red after a long drive.

I find it non intuitive that the internal engine oil reading matches to the Transmission fluid temperatures. Also that the engine coolant isn't influenced by the warmer transmission temperature or warmer engine oil temp. If there is a meta system here it seems that there would be more interaction or influence on the common factor (coolant), and that the engine oil and transmission fluid would track separately. So perhaps double check the sensor mapping?

Are the observed temps outside the 'safe' range?

I too find it non-intuitive that those temps are high and in lock step but the coolant temp is "normal". Note that the torque converter temp will go up as expected when the TC is unlocked. And actually they don't necessarily move in lock step, they're just really close (i.e. I think at one point when I was pushing 75mph the TC was reading 211, the AT pan was reading 210, engine oil was 208F or 209F, and coolant was 199F... but oil temp did move up to 210 eventually and IIRC the TC temp dropped down to 210 as well after a minute or two). They definitely will settle into the ~198-199F range on the highway when doing anywhere from maybe 40 or 45 up to 60 though. Idling in a parking lot for a bit they dropped and stabilized around 186-187F.

I don't know where the sensors are precisely located so it's hard for me to say whether the sensors are being influenced by latent heat but the fluid itself is actually OK. That feels unlikely to me, but is certainly possible.

FWIW I have no idea what's really "safe". I've never previously seen engine oil temp get much higher than about 202F - and always inline with the coolant temp - so that's part of my surprise. The A/T light doesn't come on until the TC hits >305F, and then goes off when it hits 270F, which to me is waaaay too high.

 

[linuxgod]

Oil is going to be alright at 210. you can pretty safely run up to 240 or even a hair more. You may want to shorten up your change intervals. Engine temps are good obviously. Just on principle you may want to add an external cooler. As for the front diff in your #1. Nothing you have measures either diff temperature stock. That is in the land of class 8 trucks and off highway equipment

Thanks. Yeah I know diff temp is not actually part of what I'm monitoring. I believe Rob said he'd actually added a temp gauge to his diff, but that might only be the rear. I'm inferring diff temp is hotter than normal and is the culprit given 3 other sensors read high and I went to Nitro 4.88s about 800 miles ago. It absolutely could be something else. I've done a lot of towing and I monitoring the temps obsessively so seeing a 10F jump is surprising. I realize if I drop the skids it won't actually tell me if the diff is cooling better, but if the engine oil temp and A/T temps drop then I'm inclined to believe that is the case.

If in the answer is this is normal for my aftermarket setup (4.88s + winch bumper) then an external transmission cooler would be a good idea (thanks).

 

[Whiskerz]

What we are missing is what RPM are you turning at these speeds. I am willing to bet most of the temp increase is an RPM increase.

 

[linuxgod]

What we are missing is what RPM are you turning at these speeds. I am willing to bet most of the temp increase is an RPM increase.

You'd think so but I don't think it is.

I used to tow in 4th with the factory 3.90s and would turn ~3100 RPMs at 75mph. Temps always stayed under 200F except in the mountains.

In this case with Nitro 4.88s both in 4th gear at ~3700 RPMs (with torque converter locked) and in 5th gear at about ~2700 RPMs (also with the torque converter locked up) my temps were always around 210F. So turning more or less RPMs with 4.88s made no difference in the temps - both scenarios are ~10F hotter than OEM gears.

 

[Taco2Cruiser]

I wouldn’t run 140 weight, well… I say that like you’re doing harm. You are not, I just don’t feel that it is the best option. 140 is better for low speed, banging around, like rock crawling. 90 is more suited for high speed, like towing.

I run Amsoil 110 for a good blend as I tow very heavy (8,500# and I wheel). I tried a lot, and stopped using redline diff oil. It caused a bit more wear and ran hotter than Amsoil. When I know I’ll be doing a lot of water crossings, I just run some Walmart gear oil. Bit like it’s bad, maybe I just like to spend lots of money on overkill oil.

Interesting that you see higher trans temps. The lower diff gears you go (numerically higher for the new guys) the less multiplied torque you apply to your transmission. Which will absolutely yield longer trans life. Kinda another reason why I do get why people are swapping to higher ratios.

Anywho, yeah, my temps went down a hair with lower diff gears. But that was on my truck, and I’ll be the first to say, that the 200 is not as cookie cutter as we would like it to be. They do have their variances.

 

[linuxgod]

I wouldn’t run 140 weight, well… I say that like you’re doing harm. You are not, I just don’t feel that it is the best option. 140 is better for low speed, banging around, like rock crawling. 90 is more suited for high speed, like towing.

I run Amsoil 110 for a good blend as I tow very heavy (8,500# and I wheel). I tried a lot, and stopped using redline diff oil. It caused a bit more wear and ran hotter than Amsoil. When I know I’ll be doing a lot of water crossings, I just run some Walmart gear oil. Bit like it’s bad, maybe I just like to spend lots of money on overkill oil.

Interesting that you see higher trans temps. The lower diff gears you go (numerically higher for the new guys) the less multiplied torque you apply to your transmission. Which will absolutely yield longer trans life. Kinda another reason why I do get why people are swapping to higher ratios.

Anywho, yeah, my temps went down a hair with lower diff gears. But that was on my truck, and I’ll be the first to say, that the 200 is not as cookie cutter as we would like it to be. They do have their variances.

Thanks man, good info.

I would not expect to see higher transmission temps - especially baseline temps (those on flat lands at low elevation with normal ambient temps) which seem to be up particularly as speed increases.

Yeah the weird thing to me is coolant temp is steady at 199F, but engine oil and transmission temps are all up. Previously I used to see engine oil temp mirror coolant temp (with a degree or two, at least once warmed up) but transmission temps would vary more depending on the state of the torque converter lock-up. My sense (which may be wrong) is that the transmission cooler is working fine but the fluid is getting re-heated on its path back from the radiator to the transmission pan. But why that is happening I cannot say. It's still odd to me that the engine oil temp is now closely following the A/T pan temp as it didn't used to vary.

In my case it sounds like you'd recommend swapping back to 75W-90 (particularly if you saw it running hotter than Amsoil). I may do that, but I'm curious if you think this might be accounting for the higher temp, or if it's just a side note for me to consider when I change the diff oil in 15-20k miles.

Thinking about this,,, Is there a good way to determine if the diff is the source of extraneous heat in this case? Or maybe a better question is "if you point an infrared thermometer at X point on the front diff, what temp do you see?" I have no idea how hot the front diff housing gets but if you see 140F then unless I'm cooking at 200F I don't see how it would be pushing up all the other temps significantly

I may look through the FSM diagrams later to see if there's any heat shielding above the front diff that I'm missing or if the transmission return line and engine oil cooler are real close to it or something. Overall 210F isn't awful, but my OCD says something isn't quite right and I must fix it!

 

[Taco2Cruiser]

Thanks man, good info.

I would not expect to see higher transmission temps - especially baseline temps (those on flat lands at low elevation with normal ambient temps) which seem to be up particularly as speed increases.

Yeah the weird thing to me is coolant temp is steady at 199F, but engine oil and transmission temps are all up. Previously I used to see engine oil temp mirror coolant temp (with a degree or two, at least once warmed up) but transmission temps would vary more depending on the state of the torque converter lock-up. My sense (which may be wrong) is that the transmission cooler is working fine but the fluid is getting re-heated on its path back from the radiator to the transmission pan. But why that is happening I cannot say. It's still odd to me that the engine oil temp is now closely following the A/T pan temp as it didn't used to vary.

In my case it sounds like you'd recommend swapping back to 75W-90 (particularly if you saw it running hotter than Amsoil). I may do that, but I'm curious if you think this might be accounting for the higher temp, or if it's just a side note for me to consider when I change the diff oil in 15-20k miles.

Thinking about this,,, Is there a good way to determine if the diff is the source of extraneous heat in this case? Or maybe a better question is "if you point an infrared thermometer at X point on the front diff, what temp do you see?" I have no idea how hot the front diff housing gets but if you see 140F then unless I'm cooking at 200F I don't see how it would be pushing up all the other temps significantly

I may look through the FSM diagrams later to see if there's any heat shielding above the front diff that I'm missing or if the transmission return line and engine oil cooler are real close to it or something. Overall 210F isn't awful, but my OCD says something isn't quite right and I must fix it!

So… this is where I may not be able to help much more. My gears are cryo frozen and run below 115° when towing.

Honestly, all the gears I’ve set up, once broken in, you can put your hand right in the bottom (reaching in through the side of the driver side lower control arm) and you can leave your hand there.

But I just don’t see how a diff can emit enough heat, an aluminum diff with cooling fins no less, and heat up the engine bay so much that you see a raise in coolant temps. If that was the case, you would probably be pushing engine fire temps at that point. I mean, I’ve personally experienced diffs that when broken open, ran dry under race conditions, and destroyed themselves, and still didn’t heat up a thing around them.

Anyway, anything below 212° when towing wouldn’t make me bat an eye. What I would really look at is oil temp, as the hotter that goes above 100°C, the thinner it gets. If you run 30 weight oil at 240°, it’s 20 weight oil.

other things to think about, don’t j is if you added any recently. Lights, winches, license plates, really anything you put on the front of your truck will increase heat due to lower air flow.

just trying to throw out anything that helps. But again, I just don’t see how a diff ratio change on you truck, with you set up, would be the direct reason for higher coolant temps.

reminds me when I put my own ECU in my FJC. Then on the highway I lost power. I spent two days in my tubing computer trying to figure it out. Turns out, the snorkel was plugging up with ice, limiting air flow.

 

[linuxgod]

So… this is where I may not be able to help much more. My gears are cryo frozen and run below 115° when towing.

Honestly, all the gears I’ve set up, once broken in, you can put your hand right in the bottom (reaching in through the side of the driver side lower control arm) and you can leave your hand there.

But I just don’t see how a diff can emit enough heat, an aluminum diff with cooling fins no less, and heat up the engine bay so much that you see a raise in coolant temps. If that was the case, you would probably be pushing engine fire temps at that point. I mean, I’ve personally experienced diffs that when broken open, ran dry under race conditions, and destroyed themselves, and still didn’t heat up a thing around them.

Anyway, anything below 212° when towing wouldn’t make me bat an eye. What I would really look at is oil temp, as the hotter that goes above 100°C, the thinner it gets. If you run 30 weight oil at 240°, it’s 20 weight oil.

other things to think about, don’t j is if you added any recently. Lights, winches, license plates, really anything you put on the front of your truck will increase heat due to lower air flow.

just trying to throw out anything that helps. But again, I just don’t see how a diff ratio change on you truck, with you set up, would be the direct reason for higher coolant temps.

reminds me when I put my own ECU in my FJC. Then on the highway I lost power. I spent two days in my tubing computer trying to figure it out. Turns out, the snorkel was plugging up with ice, limiting air flow.

Thanks. Haven't made any changes, aside from the gears, in a couple years. Ok not true, I actually added a single drawer to the 3rd row recently, but if that is affecting my temps then I'm gonna eat my hat.

I'm at a loss for the difference in temps as well. I wouldn't think the diff shouldn't be that hot either, certainly not hot enough to move the needle from 200F to 210F. So maybe a better question for the masses is "what would cause engine oil and transmission temps to increase by 10-12 degrees at highway speeds (70-75 mph) where coolant temp would be of limited or no change from a normal baseline?"

Honestly you're probably correct, I could run 210F or a bit higher all day long and the truck would still probably go 300k without a major issue. It just bugs me that it's different this year than it was the last 5. Like it's a sign that something is wrong.

 

[TheGrrrrr]

I'm no expert and this may be dumb logic, but I'll throw it out anyway...

I think I would do a very closely monitored/recorded replication of the conditions/change (temp) over time to ensure that the readings are dead accurate and well documented to avoid any chance that some other factor hasn't been missed. Then if everything is happening as it has appeared so far, then at least you have a confirmation of what you were seeing as well as a new baseline to work from. After that, I think I would look into what kind of friction is generated as new gears are breaking in. I would expect that if new gears shed some metal early on, as has been reported, then there must be an increase in friction during that period and therefore heat. How much? Who knows, but it must be some. I think I would also want to know what the margin of error is for the sensors and try to determine if the oil in use could have any impact on the sensor reading... then move onto the next set of variable that could result in a 1-3 degree difference, under the theory that sometime its not the one big change causing the unexpected outcome, but instead a stacking of small changes related to the big change.

Maybe there are a handful of things that add up to the differences you are seeing. My last thought would be: "If I didn't have a scan gauge telling me there was a different temp under these conditions, in what other ways would that manifest as something I would notice as driver?" If nothing comes to mind, then the next logical question becomes "Does the difference I'm seeing matter or is it within the performance envelope of the components?"

Now that I've typed all of this I realize you already understand all of this, but I'm posting it anyway because sometimes additional perspective can have unintended benefits.

 

[TheGrrrrr]

I'm no expert and this may be dumb logic, but I'll throw it out anyway...

I think I would do a very closely monitored/recorded replication of the conditions/change (temp) over time to ensure that the readings are dead accurate and well documented to avoid any chance that some other factor hasn't been missed. Then if everything is happening as it has appeared so far, then at least you have a confirmation of what you were seeing as well as a new baseline to work from. After that, I think I would look into what kind of friction is generated as new gears are breaking in. I would expect that if new gears shed some metal early on, as has been reported, then there must be an increase in friction during that period and therefore heat. How much? Who knows, but it must be some. I think I would also want to know what the margin of error is for the sensors and try to determine if the oil in use could have any impact on the sensor reading... then move onto the next set of variable that could result in a 1-3 degree difference, under the theory that sometime its not the one big change causing the unexpected outcome, but instead a stacking of small changes related to the big change.

Maybe there are a handful of things that add up to the differences you are seeing. My last thought would be: "If I didn't have a scan gauge telling me there was a different temp under these conditions, in what other ways would that manifest as something I would notice as driver?" If nothing comes to mind, then the next logical question becomes "Does the difference I'm seeing matter or is it within the performance envelope of the components?"

Now that I've typed all of this I realize you already understand all of this, but I'm posting it anyway because sometimes additional perspective can have unintended benefits.

I'm now re-reading and realizing I was thinking about the problem incorrectly as a temp issue in the diffs. My bad.

 

[grinchy]

What is the accepted logic on transmission temps? is it anticipated that 5th gear locked up is higher or lower tranny load than 4th gear lockup?

Where is it anticipated to have higher engine oil temps? Higher rpm or lower rpm? Higher engine coolant temps?

build that table and see where the data doesn’t match to expectations.

Are your trailer brakes dragging?

 

[linuxgod]

What is the accepted logic on transmission temps? is it anticipated that 5th gear locked up is higher or lower tranny load than 4th gear lockup?

Where is it anticipated to have higher engine oil temps? Higher rpm or lower rpm? Higher engine coolant temps?

build that table and see where the data doesn’t match to expectations.

Are your trailer brakes dragging?

In my prior experience if the torque converter is locked up then transmission temps generally mirror coolant temp, and engine oil temp pretty much always mirrors coolant temp regardless of A/T temps.

I’d expect higher engine oil temps at higher RPMs but in this case I see the same high temps whether I’m higher RPMs than before or lower RPMs than before. Eg I used to see 198F at about 3100 RPMs in 4th gear going 75 with 3.90s, now on 4.88s I see 210F at 75 whether I’m in 4th gear spinning 3600 RPMs or in 5th gear spinning about 2700 RPMs. Torque converter locked up in all 3 scenarios. So I don’t think it’s due to RPMs.

Trailer brakes are good, definitely not dragging.

my next step will be to drive some highway speeds in 4th or 5th gear without the trailer and see if the temps are still hot. Then try in 6th. I have a feeling they will still be this hot even without the trailer, it’s just that I don’t normally monitor without the trailer attached so I didn’t notice until I got the trailer out for the first spring trip last weekend

 

[linuxgod]

I'm now re-reading and realizing I was thinking about the problem incorrectly as a temp issue in the diffs. My bad.

No problem. In fairness it’s not clear to me if it’s actually an issue with high diff temp causing heat to “bake” other components, or if this is an unrelated issue. TBH I can’t explain why the diff would cause this but it’s been my focus since it’s the only thing I’ve changed in the last year (aside from new tires recently).

if I didn’t have a reader I wouldn’t have noticed this. Does that mean it’s not a problem? I don’t know but it is different. My sense is it might be fine but if I’m relying solely on warning lights to tell me if there’s a problem then it should be fine if my transmission is running at 300F because the A/T temp light doesn’t come on until 305F. I don’t believe anything near 300F is ever acceptable, but I’m not a Toyota engineer so WTF do I know?

 

[TheGrrrrr]

No problem. In fairness it’s not clear to me if it’s actually an issue with high diff temp causing heat to “bake” other components, or if this is an unrelated issue. TBH I can’t explain why the diff would cause this but it’s been my focus since it’s the only thing I’ve changed in the last year (aside from new tires recently).

if I didn’t have a reader I wouldn’t have noticed this. Does that mean it’s not a problem? I don’t know but it is different. My sense is it might be fine but if I’m relying solely on warning lights to tell me if there’s a problem then it should be fine if my transmission is running at 300F because the A/T temp light doesn’t come on until 305F. I don’t believe anything near 300F is ever acceptable, but I’m not a Toyota engineer so WTF do I know?

Everything about this seems odd. I think I would set aside the changes you made and focus first on why the engine and transmission temps would be running higher but not the coolant. That seems like the best place to start. From there work back towards possible root causes?

In an immediate reversal of my thinking, it seems like tires could change the load on the drivetrain, so I suppose that could be a source.

Me guessing about things I don't fully understand isn't going to help, so I'm just going to hang out and watch you and Rob figure it out because I agree that understanding the change is the only option. Ignorance is NOT bliss. My problem is that if I were encountering this situation with my truck, I'd be tagging you, Rob and @TeCKis300 for help so...

 

[bloc]

Fan clutch? Maybe the extra RPMs are keeping it in a place it is less happy? And there is enough extra cooling capacity to manage the water temp but for instance the thermostat is having to stay open longer than before to keep engine temp ideal, which means water temp at the bottom of the rad will be higher than before.
I’m not sure how this would impact engine oil temps.. do we know whether the oil cooler is bathed in the main head coolant flow or the constant/bypass flow for the heater core?

 

[TeCKis300]

Interesting symptoms. If I had the reach based on the provided info, I'd suspect the following items, or a combination there-of. Riffing on thoughts brought up by @Taco2Cruiser.

Agreed diff oil viscosity is too high. There's honestly no reason to go that high. Gears already provide you additional mechanical advantage and the diff's not going to be working harder. Just the opposite as gear mesh pressures are lower and oil circulation increases with greater input pinion rpm.

There's probably several things going on with the diffs:
1) Increased viscosity and the resulting fluid frictional losses
2) Higher pinion input rpm relative to vehicle road speed, further magnifying viscosity frictional losses with oil slinger rpm
3) Freshly built diff that's "tight"
4) Multiply the effects of 1-3 by two, in the other diff

I think the engine and transmission is just working incrementally harder against efficiency losses in the diffs. The data reads to me that way as it increases linearly with vehicle speed. Independent of engine RPM or transmission gear. Coolant may not be a great indicator here - we know the engine has to produce more HP to sustain a higher cruise speed, yet the coolant temps remain steady and independent of output.