rl, please consider this as comment rather than education...rubberlegs said:Something about these high tire pressures doesn't make sense. Educate me. Here's my thoughts:
Or replace all those complicated steps with the chalk test...
- Stock tires, placard pressure is 35 psi in our truck.
- Then we add a heavy camper on the back. Front axle load barely changed, 2700 lb. Rear axle was way lower, but went way up to 3300 lbs.
- OK, leave the front pressure at 35 psi. Up the rear pressure by the ratio 3300/2700 = 43 psi. (Stock tires rated at 51 psi max)
- Now replace the stock tires with E-rated. Why would the pressure change to such big numbers? The load is really carried by the air. And the E-rated tires are probably stiffer (likely a second order effect), so maybe even lower the pressure.
I like using the TRA (Tire and Rim Assoc) tables. That's where I found the "max loads at a given psi" figures I referenced, post #4 (doc link within) and I use them for my own tires. Good thing is, TRA is the industry standard for Can/US and the commonly available mfrs are members.
Re your calculation, note that according to the tables the math is not always linear. eg for P245/75R16 max load at 32 is 2172#. increase the psi by 28% to 41 and the stated max load is 2469# - not the expected 2780#
I picked a P metric size because that's all I found listed for the Tacoma I looked up. I didn't notice any P metric size in the TRA tables showing a load at 51 psi but I know you mentioned "stock". Point is, if changing from P metric to LT it's good to reference the tables to ensure the new starting psi is adequate. This is covered in the Guide (same doc, see pg 11).
Note also that P metrics are derated by ~10% when used on a light truck.
In the Guide example given, P metric to LT meant going from 35 psi to 45 psi. So, again, good to watch the math. [ON EDIT so both the psi and the stiffness of the tire construction increases. Oh, well...]
Finally, when shopping for different tire sizes the tables are handy. I was able to find a size that is rated for ~10% more load at a given psi than the OEM size. So for a slightly larger size, that still fits on the OE rim, I can run the replacement tires at a ~10% lower psi. Which for me, is a very good thing.
As for the high pressures and making sense, I've already said my bit. What hasn't been mentioned is uneven tire wear tire service longevity - if dollars and cents matter.