Tire Pressure Question - E vs. G Tires

[Jopopsy]

Hey Gang,

So, I’ll admit to being stumped. I must be missing something obvious, but I don’t know what it is.

I’m getting my head around the tire pressures for the rig I have on order. I’ve never had ‘G’ rated tires before; my only non-passenger tire experience is with the Class ‘C’ rigs I’ve had in the past. Those rigs run on ‘E’ rated tires.

So, in my prior Greyhawks the Ford placard called for 75 lbs in the front and 80 lbs in the back. The FAWR for the E450 is 5000 pounds, the RAWR is 9600 pounds. So, pulling up the load charts for 225/75R16 E tires I see 5120 pounds for single front (single) and 9880 dual rear (dual). Fair enough.

Now, skip to the Precept. The Ford placard I found here in Mark’s thread calls for 90 pounds front and rear. 8000 pounds on the front, 15000 pounds on the back. The load chart for 235/80R22.5 G calls out 4140 for the front (single), and 7530 for the back (dual).

Those make sense to me, 4140 * 2 is just over the FAWR 8000 pounds, and the 7530 * 2 is just over the 15000 RAWR.

However, the ‘E’ rated load chart didn’t require me to multiply by 2 to satisfy the numbers.

What am I missing? Its driving me crazy - the analytical side of my head can’t make peace.

[Jopopsy]

Another observation I just made.

The load tables for the ‘E’ rated tire show the following:
Max load & pressure on sidewall
Single Tread 2680 at 80 PSI
Dual Tread 2470 at 80 PSI

These numbers, when multiplied, give you the max weight for the tire at 80 pounds. 5360 (2680 *2) and 9880 (2470*4).

Flip to the ‘G’ table:
Max load & pressure on sidewall
Single Tread 4675 at 110 PSI
Dual Tread 4410 at 110 PSI

This lines up directly with the max weight of the tires at 110 pounds. 4675 is the max weight at 110 pounds for a single tire, and 4410 * 2 is the max weight at 110 pounds for dual tires (8820).

So I ‘think’ I answered my own question, but it begs another question entirely. Why do the charts handle the weights differently? I can see somebody getting themselves in trouble if they don’t read carefully or make assumptions. It seems the ‘E’ chart is skewed to giving you weights across the axle, the ‘G’ chart is skewed to giving you weights according to tire (or more accurately each ‘corner’).

[RogerR]

Based on my understanding of motor vehicle tires the mfg. plate is intended to for a blend of safe handling and comfortable ride. Running higher grade tires at higher pressures results in a rough ride and potentially unsafe handling characteristics. Tires inflated beyond vehicle mfg. suggested psi may not have good contact with the road for steering and braking. I would not run the higher pressures unless I overloaded the vehicle.