Different Sized Tire (Left vs. Right) Emergency Spare Situation, Please Advise (1 Viewer)

Hi folks, well, i went and wheeled at slaughterhouse gulch today ( about 5 to 6 on a scale of 10) and managed to slash the sidewall on one of my super new, super cool 285-75-16 REVOS.

Thank goodness i have the free replacement warranty and thank goodness i verified with the tire store that the free replacement warranty covers offroad usage.

From many moons ago my spare tire sits somewhat smaller at 265-75-16.


(((Ignore this part if you want ::: The spare tire size was not something I really paid any attention at all to for three reasons: I regularly commute less than twenty miles a day. When I commute to company meetings i'm hitting highways that are so crowded, fast, and cramped it makes changing flats suicidal and so in this event I would simply stay in the vehicle as far off the road as possible and rely on my AAA membership to flatbed tow truck me to a tire store. And lastly, one of the guys I usually wheel with has the same size tires and wheels as i do and he normally carries two spares so we've joked that he can blow out three tires if he uses my spare, and likewise, i can blow out three tires if i use his spares! Ohh, BTW I only went through that cause someone was bound to slap me with the "why dont you have a same size spare question. Still, I'm stupid to not have the same size spare. , on me! )))

So, my question is how much harm did I do during driving home? I loped along at no more than 45 with an average of about 35. I stopped at every twenty mile interval on the way home and felt the transfer case / viscous coupler. It was warm to the touch and at times toward the end hot to the touch (you could hold your hand on for a good twenty seconds before the pain was too much) but it was never too hot to touch. I drove a total of 110 miles from flat to home. After about three "twenty mile stops" I found a self-serve power wash and let the truck sit for about thirty minutes and then thoroughly power rinsed the truck making sure not to shock anything with direct spray but instead making sure to sweep the spray all over the undercarriage. I found another wash at around another forty miles and did the same rest, wait, rinse.

My other question is more fundamental: I just cannot figure out why it would matter whether the tire diameter was different on one side vs. the other. Well, within a reasonable difference that is. (((BTW, how much different is a 265/75 vs. a 285/75?))) I mean any time you take a corner one tire spins at a different speed then the other which is effectively the same thing that happens when different diameter tires are on. I guess doing this for extended times (as in straight driving on different diameter tires ) is a different deal maybe. BUT then, even then, why would it matter to the TC / VC???? I found a thread specifically on different size front vs. rear and in that case I can totally see how the VC would work really hard but I just cannot see how it would matter that much if the different size were on the same axle? Yet, I have heard from so many sources that driving different diameter tires with a full time four wheel drive is really rough on things. In fact, so far, no one has told me that it is okay at least for short trips.

So, wise ones, what am I missing here? Someone please prove I'm a dumbass for worrying about this cause in that process you will help me to stop worrying about it!!! Any advice at all is greatly appreciated.

Okay, I answered one of my questions:

285-75-16 = 33"

265 - 75 -16 = 31.5"

So, the difference between left and right on one axle only was 1.5". Still not sure about the rest of it. Thanks.

Turbo,

Only one rear tire being a different diameter than the fronts is not as big an issue for the viscous coupler is that it halves the rotating speed differential between the front/rear shafts versus having BOTH rear tires a differenti diameter. In a nutshell, the rotating speed of the rear drive shaft would be the AVERAGE of your correct size tire and the undersize tire. So, this was not as bad as running a pair of undersize tires on a given axle.

In your case, you don't have a thing to worry about whatsoever. I think even if you had 2 undersize tires you'd be fine if you simply blew home at 65. The unit is tougher than that and can handle a couple hours of abuse in stride. Smart move doing exactly what you did, however. Letting it cool, feeling the TC, and even taking action to actively cool the unit - all very good procedure to protect a piece of machinery that will in turn provide you with years of service.

If you wanted to be very anal about it, consider draining and refilling the center differential in case there were hot spots here and there that degraded the oil's additive packages. This is a textbook case why we should all have fresh fluids in place, as fresh gear oil can handle a lot more abuse in a semi-emergency situation. Your actions were also a textbook example of how to handle this situation and protect expensive machinery.

If you have many miles on your Revos, there may be a significant size difference you may want to address with your dealer since you have a full time vehicle. Toyota does not publish an allowable tire size difference to my knowledge, but Subaru and Audi only allow a .75 inch CIRCUMFERENCE difference.

DougM

IdahoDoug said:

Turbo,

Only one rear tire being a different diameter than the fronts is not as big an issue for the viscous coupler is that it halves the rotating speed differential between the front/rear shafts versus having BOTH rear tires a differenti diameter. In a nutshell, the rotating speed of the rear drive shaft would be the AVERAGE of your correct size tire and the undersize tire. So, this was not as bad as running a pair of undersize tires on a given axle.

In your case, you don't have a thing to worry about whatsoever. I think even if you had 2 undersize tires you'd be fine if you simply blew home at 65. The unit is tougher than that and can handle a couple hours of abuse in stride. Smart move doing exactly what you did, however. Letting it cool, feeling the TC, and even taking action to actively cool the unit - all very good procedure to protect a piece of machinery that will in turn provide you with years of service.

If you wanted to be very anal about it, consider draining and refilling the center differential in case there were hot spots here and there that degraded the oil's additive packages. This is a textbook case why we should all have fresh fluids in place, as fresh gear oil can handle a lot more abuse in a semi-emergency situation. Your actions were also a textbook example of how to handle this situation and protect expensive machinery.

If you have many miles on your Revos, there may be a significant size difference you may want to address with your dealer since you have a full time vehicle. Toyota does not publish an allowable tire size difference to my knowledge, but Subaru and Audi only allow a .75 inch CIRCUMFERENCE difference.

DougM

Click to expand...

Doug, thanks so much man on so many levels; that was informative, helpful, kind and logical.

Again, I have heard from so many sources that any tire size differences on an AWD / Full Time 4WD can be catastrophic and yet the whole way home I kept thinking to myself over and over why the VC would care with the situation then! Your answer about it averaging the tires' sizes makes perfect sense and I can probably calculate that by taking the 33" and 31.5" measurements, averaging, converting to circumference, comparing that to the circumference of the two 33" front tires, and then calculating for my 110 miles exactly how much more the front driveshaft turned than the rear. Anyway, thanks, that made perfect sense.

I'm extra glad to get the endorsement on my rest, wait, rinse process!!! I felt so anal about that; all my friends from the trail INSISTED on staying with me through the whole thing (which is what friends do i know) but I was asking them each time to let me ride my horse home alone OR hit a restaurant or bar on me for each of the rinse sessions. Nothing doing, they all stayed as loyally as possible as my anal self rinsed the rig over and over!!! I felt so stupid but now feel somewhat smarter about what i did.

The funny thing was all that feeling of the VC was meaningless; I had never really felt it for heat before so I had no baseline data point for it. It was just part of my anal guess-work while working out why the frickin tire size mattered so much.

I was going to change the fluid in both the TC and the rear diff (might also do the front only cause i dumped it in some mud soup). All three fluids have about 8K on them now and it is all M1 so in terms of additive packages and such it should be strong.

Thanks again man, you really reassured me and I really appreciate it. I'll be taking the tire to replace today and I'm hoping the tire store is as willing to do it now as they were when they sold the warranty to me. They specifically and repeatedly advertised offroad use does not void the warranty whatsoever. So anyways, thanks again man.

Did A Little Math

Okay, tried to work this out:

Circumference = (3.14)D where D is diameter.

From ID's posts above the transfer case / viscous coupling would AVERAGE the difference between the right and left tires, so:

FOR REAR AXLE SPARE SITUATION:

33" tire + 31.5" tire = 64.5" total tire size

64.5 / 2 = 32.25" AVERAGE DIAMETER REAR AXLE

(3.14) * 32.25 = 101.265" AVERAGE CIRCUMFERENCE REAR AXLE IN INCHES

101.265 / 12 = 8.438' ACRA IN FEET


FOR FRONT AXLE:

33" tires left and right equals 33" average diameter tire.

(3.14) * 33 = 103.62" AVERAGE CIRCUMFERENCE FRONT AXLE IN INCHES

103.62 / 12 = 8.635' ACFA IN FEET


With the length of a mile being 5280 feet:

The average front tire rotated 611.46 times per mile.

While the average rear tire rotated 625.74 times per mile.

The difference per mile is 14.28 revolutions between the front and rear axles average tires. So after 110 miles my rear axle's tires rotated approximately 1570 more times than the front axle!

So what does this mean, well, duuuhhhh, it means I'm gonna drive exactly 2.567 (1570 / 611.46) miles with the rig on jackstands, the rear driveshaft removed and the stereo blasting to recreate the same conditions we went through when returning home!!! Yes, OCD sucks, well at least I can clean and lube my DS while it's out! Yes, Just Joking, and yes, I admit I actually considered attempting this (for a few seconds just long enough to bend my knee backwards fast and hard enough to kick myself in the ass!)

Ohh, thanks again for the advice Doug, now quick recommend a psycho-therapist!!!

woah, way too much writing on this thread, but I guess it's needed.
I'll give you my real world experence here. I have a 31" spare and I ride 33". I've used my spare a few times. Nary a problem.
I've even put a single 35 (the rest were 33's) on my front just to see what it loked like. It looks nice by the way,see link.
http://www.cardomain.com/ride/707497/6
I took it like this to the empty lot beside my home but I DID NOT LOCK THE DIF.
I'm not scientific, but I know what I know and for me, it works.

Glad to be of service. BTW, Eric makes a good point - never lock any diff when using a different size spare unless you're on a trail where some slippage can occur. Sparingly even then.

DougM