Larger tire for lower psi and better ride?

[highfructose]

are p rated tires at 33 psi “harder” than lt rated tires at 40 psi?

I’m having a hard time getting my head around why the Oem 285 50 r20’s call for 33 psi while heavier duty 275 60 r20 lt tires need 40 psi…and why a 275 65 r20 needs 36.

I’d like the “squishyness” of my tire be as close to oem as possible so the suspension “flexes” the way it was designed.

Do I need a p rated 275 60 r20 so I can run at 27-33 psi? Or will that not be any more squishy/compliant than a 275 60 r20 lt tire?

 

[bloc]

Heat is the reason.

The thinner carcass of a p metric tire doesn’t generate as much friction within the structure of the tire for a given pressure, so it can be run at a lower pressure and flex more without going into self destruct mode.

The more robust LT metric tires need more pressure because if they flexed the same amount while rolling down the road they’d generate much more heat and eventually start to come apart.

This is the primary reason the load rating of a tire changes with the inflation pressure. That pressure is the minimum needed to carry the rated load at the rated maximum speed without overheating.

The increased pressure required of LT tires is a primary reason the ride quality changes. Put your P-metric tires at a similar pressure and you can experiment with the difference.. especially over sharp road imperfections. Then, with the generally heavier LT tires also having stiffer sidewalls and tread area, the ride on those will be even worse, if marginally.

If you really want to keep stock ride quality you need to run a P-metric or SL/XL load tire that allows similar pressures to stock. Avoid LT. And note that if you run a significantly larger sidewall with stock pressure, that will also really impact handling dynamics through sidewall flex.

 

[Heckraiser]

While all of the above may be true, you are not going to destroy an LT tire by running lower PSI. It's load handling capabilities will be reduced and tread life will also be reduced (either by under-inflating or over-inflating), but there are tons of people out there running E-rated tires in the low 30's to produce the desired ride quality. I run the e-rated 315's on my 80 at 32psi at all times except when towing, and have for years over tens of thousands of miles and have not seen any evidence of harm other than a potentially lost mpg. Many guys in the 80 forum run similar pressures. I've also had great luck dropping the pressure in the 285/75r18 on my LX570 recently.

Tire manufacturers recommend 40psi on most LT tires, but that doesn't mean the world is going to end if you drop it to whatever pressure produces your desired ride/handing. Just try not to go below 30 on the pavement and see what pressure floats your boat. The word "need" (i.e. such and such tire "needs" 40psi), is not entirely appropriate to the application.

 

[bloc]

The word "need" (i.e. such and such tire "needs" 40psi), is not entirely appropriate to the application.

Yes there are engineering overheads, plus in this application or most 80s we aren’t going anywhere near the load rating per tire, not in the worst case road type/temp/etc conditions they engineered for either. I ran my 315s at 33 or so around town but usually a bit over 40 on the freeway to save fuel.

Which brings up… the increased heat generated from friction is being supplied by energy from somewhere. In our case that’s fuel. Dropping tire pressure will decrease fuel efficiency even more than the hit you’ll already get going to a LT from a P metric or SL/XL tire. It won’t be an additional 5mpg but it does add up

@highfructose LT tires being in the 40s is an attempt to match the stock tire load rating at 33psi. Even that is well above what we will realistically put on that tire as far as load. What @Heckraiser is pointing out is even at less pressure (to a point) the tire won’t suddenly grenade because we’re still technically in the operating window the tire allows. Keep your tires in there and you should be safe to experiment with things.. just know there may be mileage, or road holding, or dynamic suspension compromises too.

 

[TeCKis300]

Direct from the Application of Load Inflation Tables document:

This supports @bloc 's comments. It's about managing and staying within the heat threshold of the LT tire carcass for load capacities up to its full performance envelop, which includes max speed. More material that composes an LT tire generates more heat. It's a selfish parameter of the LT tire architecture with respect to only load capacities. It does not address any tuning for a vehicle.

On that last point of tuning to a vehicle and the balance of PSI for load against that tuning...

It also goes to state that the higher PSI requirements may not be suitable. Which has been my beef with RCTIP. To @Heckraiser 's point, there needs to be some pragmatic balance and common sense applied.

 

[grinchy]

The cold to hot tire pressure readings can help one get a feel for what is acceptable’heating’.
I’ve read that 10% increase is a good target to aim for.
I’m currently running 34 cold to 41 hot though .
My current tires seem to top out at 41 hot, whether running any pressure between 34 and 37. I like the feel of 34 more.

 

[Leaky2014]

I can say that in my experience with K02's LT E's that 45psi is more comfortable than at 35psi, quieter too.
I didn't have any issue with the wear running them at 35psi, but I don't put a lot of miles on it, they will age out before they run out.
Highest I run them is 50psi when towing about 7k.

Most likely will not get E rated next time.

 

[highfructose]

I can say that in my experience with K02's LT E's that 45psi is more comfortable than at 35psi, quieter too.
I didn't have any issue with the wear running them at 35psi, but I don't put a lot of miles on it, they will age out before they run out.
Highest I run them is 50psi when towing about 7k.

Most likely will not get E rated next time.

I’ve noticed that my k02’s, which call for 40 psi, are noticeably louder at 35 psi.

 

[TeCKis300]

I'd wager this has to do with how much of the tread pattern is engaged on the road at differing pressures. Specifically whether the more aggressive shoulder tread blocks are touching the road. This classically affects traction, tire wear, and interestingly to your point, noise.

Would be worthwhile to do a chalk test:

 

[highfructose]

I'd wager this has to do with how much of the tread pattern is engaged on the road at differing pressures. Specifically whether the more aggressive shoulder tread blocks are touching the road. This classically affects traction, tire wear, and interestingly to your point, noise.

Would be worthwhile to do a chalk test:

View attachment 3039274

That extra patch and noise is also probably why there’s extra heat…more tire slapping/frictioning the ground during the drive.

at 35 fr and 38 rear, the tires aren’t nearly as loud and the “bulge” is about equal front to back.

After driving a while they all get up to 39-41 psi and I end up with that slightly harsher ride I am trying to get away from.

Currently I’m torn because I’m aware that the lt tires are less likely to get punctured…but the p metric tires allow a lower psi and thus are, ostensibly, more comfortable (in 275 60 r20).

Interestingly there is an xl (non lt) tire in 275 60 r20 from nitto terra grappler that calls for 34 psi (almost same as 33 psi oem) and would, potentially, bridge the gap between sl and lt, both in psi and and in durability.

A problem is terra grapplers have a bad reputation and, interestingly, are lighter weight than most of the sl tires in this range (falkens are more than 10% heavier for instance).

This leaves me wondering whether, rather than relying on sl vs xl tire designations, I might look at tire weight and reported construction to see what p metrics might have durability more like an lt

Note: I want a tire heavier duty than stock tire partially because I’ve got a big steel bumper and a load of tools in the back that places me always heavier than stock…and the sl tires in 275 60 r20 call for like 26-27 psi, which leads me to believe that the lower psi will affect the designed/intended handling dynamics of the vehicle negatively.

 

[bloc]

and the sl tires in 275 60 r20 call for like 26-27 psi, which leads me to believe that the lower psi will affect the designed/intended handling dynamics of the vehicle negatively.

Yes. According to the load charts I could run my P-metric 285-70R17 Toyo AT@s at 26 or 27 psi for the same load rating as stock but I was basically driving around on half-melted marshmallows when I did. Not good.

You are on to something with comparing weight.. but that has caveats too when different manufacturers put more heft into the tread (will generate more heat) or sidewall (less) as an engineering decision.

Whatever you do, if you look at the load charts even the stock P tires handle well over what is needed for your bumpers and such. As has been eluded to toyota didn't design things where the tire is inflated to exactly that axle's gross rating, they added more pressure to hit a sweet spot they determined for tire structure while handling and braking, mileage, noise, wear.. a ton of things to consider. It is reasonable to say you could add a few PSI to stock tires to restore the conditions toyota intended to account for the weight you added.. but we'd all be making guesses as to exactly what those conditions all were.

As for your decisions around LT vs non for puncture resistance vs noise/heat/ride.. lots of people have had that debate, and had different priorities. Just a matter of determining your priorities. You changed the truck from stock.. reasonable to conclude that it will never ride exactly like stock with all that stuff on there.. just gotta find your sweet spot.

 

[linuxgod]

While all of the above may be true, you are not going to destroy an LT tire by running lower PSI. It's load handling capabilities will be reduced and tread life will also be reduced (either by under-inflating or over-inflating), but there are tons of people out there running E-rated tires in the low 30's to produce the desired ride quality. I run the e-rated 315's on my 80 at 32psi at all times except when towing, and have for years over tens of thousands of miles and have not seen any evidence of harm other than a potentially lost mpg. Many guys in the 80 forum run similar pressures. I've also had great luck dropping the pressure in the 285/75r18 on my LX570 recently.

Tire manufacturers recommend 40psi on most LT tires, but that doesn't mean the world is going to end if you drop it to whatever pressure produces your desired ride/handing. Just try not to go below 30 on the pavement and see what pressure floats your boat. The word "need" (i.e. such and such tire "needs" 40psi), is not entirely appropriate to the application.

It depends just how low the pressure is. Everyone remember the Ford Explorer/Firestone debacle? At some point tire pressure does in fact matter.

I think gaijin has been in several lengthy discussions in which a ~25% safety margin was calculated/assumed in the RCTIP numbers. So if your recommended pressure is 40psi and you're running at your GVWR then to your point you probably won't (quickly) destroy your tires at 32psi, though they will likely wear faster or unevenly. And as you point out if you're under GWVR then you could theoretically adjust your pressure down to compensate and end up running under the RCTIP without issue.

FWIW I've run ~18psi at <45mph on pavement for 10-15 miles between trails and my tires have been fine, but at those pressures I'm the captain of understeer around corners and on winding roads (and I would not recommend sailing that ship in any other circumstances)

 

[TLCLVR]

Wish I remembered the source, but an off-road tire guru recommended using the RCTIP numbers as a starting point and then using chalk tests and watching pressure increase to fine tune the pressure in your tires. After 20-30 min at highway speed you should see about 4 psi increase for P-metric and 6 psi increase for LT's. Any more than that and you are generating too much heat and need to increase pressure. I find this rule of thumb especially helpful when at full load or towing (esp running LTs) to find the right pressures and ensure safety.