Originally Posted By: AITG
I got the Big Dog (2003 Chev Dmax ECLB) weighed today.
Total weight: 7700 lbs.
Front axle: 4380 lbs.
Rear axle: 3320 lbs.
That means each front tire carries 2190 lbs. and each rear carries 830 lbs.
The Goodyear inflation table puts the fronts between 65 and 70 psi. The rears would be over-inflated at 35 lbs.
How soft do I dare run the rears?
There's a multi-level problem here:
First is making the assumption that if you use the table, the tire will wear flat. Nothing could be further from the truth.
Tire wear is very complex. There are a lot of things that affect it. In particular, steer tires tend to wear the shoulds and drive tires tend to wear the center. That's why it is important to rotate your tires on RWD drive vehicle - you wear different portions of the tire and get more miles!
But tire engineers will design a tire to wear evenly at typical load/inflation combinations - and that has changed lately. Many years ago, the load table was used as is, but the Ford / Firestone situation a few years ago pointed out that this was a poor enginering practice. Overdesign/ under-utilize should be the norm for components.
So the way to do this is to find the GAWR's (Gross Axle Weight Ratings), compare them to what the tire load table says the max load is at the placard pressure. The difference would be the "Reserve Load".
Then using the actual weights, add the Reserve, then look up in the table for the corresponding pressure.
HOWEVER: Because of ther recent changes in the way this is done, older pickups were done the old way, but tires are now done the new way. So I would recommend that if the reserve isn't at least 15% of the GAWR (that is, the tire should be inflated to 115% of the GAWR), then THAT value (15%) should be added in when figuring the inflation.
But here's another part of the problem: The vehicl;e's spring rates, sway bar sizes, and shock damping were all tested by the vehicle manufacturer using the placard pressure. They tested it both fully loaded and empty. Changing the pressure split changes the handling balance front to rear - and that might have negative affects. So I urge extreme caution. This is not an area to be cavalier about!