P-metric vs LT - inflation pressure - Ugh!

Tire Pressure – Who would have thought it would be this complex!

My Taco came with 265/70R16 tires and a factory recommended pressure of 30 psi per tire. I swapped them out for pizza cutters, LT235/85R16 which are load range E, max 80 psi.

I did the chalk test last night. Starting at 50 lbs I reduced tire pressure until I first saw an even contact patch. As you would expect, the higher pressures caused the chalk in the center of the tread to erode before the chalk at the shoulders. I finally achieved an even contact patch in the front when I was down to 30 psi but the back was still showing more center wear than shoulder wear. Going by chalk test only, I’d be running 30 psi in the front and probably 25 psi in the rear.

But then there is the load range table.

The OEM p-metric tires would support a load of 2045 lbs. at 30 psi in a truck car application. The new LT tires support 2030 lbs. load at 45 psi. So to have the same load capability, I’d need to run 45 psi. Dropping to 30 psi reduces the load rating to ~1530 lbs per tire.

I estimate my truck is 1350 lbs per tire in the front and 900 in the rear. When towing my 4000 lb travel trailer I’d guess that goes to ~1300 front and ~1200 rear per tire.

Summary:

1. P-metric: 30 psi, 2040 lbs load, even contact patch.
2. LT: 45 psi, 2030 lbs load, center contact patch.
3. LT: 30 psi, 1530 lbs load, even contact patch.

What pressure would you run? Take the easy way and just run 30 psi per the door sticker? Run 45 for equal load capability? Split the difference at 37 psi? Run 30 until you’re towing then fill the rear to 40?

 

Why not just run 25 until you're towing then increase the pressure? This is what most big trucks do. You set the tire pressure based on how loaded the truck is.

 

That is the direction I am leaning I may also do a temperature rise check to confirm I am not too low at 30 psi.

 

Pick a number that gives you a decent ride, decent gas mileage and makes you happy. Adjust it when you need to.