Oversize tires and pressures on- and off-road

[greiswig]

Recently installed a set of Cooper Evolution M/T's, size LT275/70R18, and I have asked Cooper to give me a table (assuming they have it) that gives recommended on-road inflation for a given weight on the axle. These are pretty tall, but narrow (9" tread width).

I'm wondering what others are doing for their pressures for on-road and off-road if they have larger than stock tires, particularly with aggressive treads? Have you found the perfect sweet spot? How'd you get there?

 

[CRYA]

I've used this resource in the past. It may or may not be helpful but has lots of good info:
https://www.toyotires.com/media/3729/application_of_load_inflation_tables_20200723.pdf

 

[djkaosone]

It's all trial and error. Some will say that you'll "need" to air down to 20-25 psi for rock gardens, blah blah blah for blah blah blah. Take it all with a grain of salt, because your truck is carrying a different weight load than they are, plus you're running different tires too.

THE best advice someone gave me was to place your truck on a rock that was the size of my head and your tire should bulge out a little on top of the rock, that's for rock gardens.

My suggestion is start with 25 psi up front and 30 psi in the rear. Then do your own assessment as you trail ride on whatever terrain you're going through.

Why the offset in psi? Look at your tire pressure sticker in your door jam. It's recommended that way by manufacturer, because of the heavier load in the rear. Well that's how I see it.

As you can see the front and rear sidewalls get used up pretty well. I use this setup for almost every terrain.

On everyday pavement I run 40 psi up front and 45 in the rear. Oem is F38 and R42, I just rounded it up. Btw... tire shops recommend oem psi for any tires mounted.

 

[greiswig]

@djkaosone I have some rules of thumb for how much to air down offroad from street pressure, but the recommendations for that street pressure vary a lot. I've had limited success with the chalk technique before, but I might have to try it again. Another one that used to be recommended was that you wanted a credit card to just barely be able to wedge under the blocks at the edges when parked on concrete.

Nice air-up setup, BTW!

 

[CRYA]

Why the offset in psi? Look at your tire pressure sticker in your door jam. It's recommended that way by manufacturer, because of the heavier load in the rear. Well that's how I see it.


View attachment 13076 View attachment 13077

The reason is the sticker in the door jam applies to the size and load rated tire from the mfr. A 255/55/19 is going to support different weights at different pressures than a 275/70/18. So to be super-precise about it, there exist load tables, such as what I was referring to above. The tables show you exactly what pressure your non-oem tires need to be to support the factory load because relying on the sticker isn't accurate anymore.

 

[Nechaken]

Recently installed a set of Cooper Evolution M/T's, size LT275/70R18, and I have asked Cooper to give me a table (assuming they have it) that gives recommended on-road inflation for a given weight on the axle. These are pretty tall, but narrow (9" tread width).

I'm wondering what others are doing for their pressures for on-road and off-road if they have larger than stock tires, particularly with aggressive treads? Have you found the perfect sweet spot? How'd you get there?

Super interested to see what they come back with.

For my 265/65/18, I've set mine to 46F / 51R for on-road use. Kind of a ball-park based guess based on other people's suggestions, and it doesn't seem out of line with estimating the truck's weight and taking a percentage of the tire's full load capacity at 80 PSI (they're E-range).

 

[greiswig]

I saw that, @CRYA . The problems I have with tables like that:

1. The relationship between pressure and the ability to support weight isn't linear as far as I can tell. So extrapolating DOWN below 35PSI (where all these tables seem to bottom out) is iffy.
2. I can't tell whether those tables refer to the ideal pressure recommended to support that weight for that particular tire, or the maximum load that should be on that tire at that pressure, or what.

 

[greiswig]

Super interested to see what they come back with.

For my 265/65/18, I've set mine to 46F / 51R for on-road use. Kind of a ball-park based guess based on other people's suggestions, and it doesn't seem out of line with estimating the truck's weight and taking a percentage of the tire's full load capacity at 80 PSI (they're E-range).

They came back with a table similar to what's been discussed above. Don't know how to interpret it, and it doesn't have load values that would map to my per-tire weight.

46/51 seem pretty high to me; ordinarily, going up in size means you go down in pressure to maintain a similar contact patch. Is your tread wearing more in the center?

 

[Michael Gain]

I've always used the 4psi rule. It's a little time consuming, but, I feel, it gives the best balance between inroad comfort and utility.

https://www.tyrereview.com.au/tyre-...xt=Let's look at how to,on your first attempt!!

 

[greiswig]

Interesting, @Michael Gain ! I've not heard of that before. I wonder what science/data supports that contention that 4PSI gain after an hour is the sweet spot?

Regardless, Cooper seems to be one of the companies that subscribes to that.