1994 E-350 Club Wagon XL, 7.3L IDI diesel, with 245/75R16 tires (BF Goodrich All-Terrain A/T). Tire max inflation pressure is 80 psi. Label in driver's door recommends 65 psi front / 80 psi rear at max rated load. At the moment, I'm driving this thing almost unloaded. Do I inflate as per the label, or less than that? If so, what?

 

Run your rear tires at 80 psi. I have run my tires at that pressure from the day we got it and the tires wore great. my door sticker says 80psi but nothing about max load. The van rides good and is stable. IMHO I won't run any less air in them

 

I run the max pressure allowed by the tire, this is proper inflation. It will help mileage, ride , and wear.

 

The max pressure allowed by the tire and the usage inflation pressures can be, and are in most cases, two different things. For the E150's the max tire pressure for the P235/75R15XL is 41 psi, this coincides with the usage pressure (shown on the door jamb, or in later models on the gas tank door). I've seen some variations in E350 pressures, but the most recent is 55 psi front and 80 psi rear. (Front pressure may be slightly higher on some models, maybe with the diesel?)

80 psi is the max recommended inflation pressure for the LT245/75R16 LRE tire (same with every LRE tire). My recommendation would be to inflate to the usage pressure that is printed on the certification label of the vehicle, as I mentioned before, usually on the door jamb.

It is important to note, vehicle manufacturers will look at a few things to determine what inflation pressure to recommend as usage. 1) What is the max load that the vehicle can carry on each axle (front and rear) and what inflation pressure do I need to carry the load + some safety factor. 2) What pressure makes my vehicle handle safely 3) What pressure makes my vehicle ride the best. By focusing on these items, in this order only, will the usage inflation pressure be determined.

Long story short: You should be OK with recommended pressures shown on the drivers door.

 

Unloaded you can run 65 in the rear. It will definately help the ride and have little effect on tire wear.

 

Rust bucket, remember how the explorer got nicknames the exploder? It was because at 25PSI (tires rated at 40) the tires were not properly inflated. I run my LT235/75/15 at ~50psi.. (PSI rating found on the sidewall) this sounds a little extreme, but I see a 2mpg increase with them at 50 instead of 41. It depends alot on the tire too.

 

pfogle, The P235/75R15XL's are rated at 41 psi, the LT235/75R15's are rated at 50 psi, so you are correct. 50 psi is the correct pressure to use. Also keep in mind that the P235/75R15XLs are rated to carry more load at 41 psi than the LT235/75R15s at 50 psi (2183 lbs @ 41 psi for the XL vs. 1985 lbs @ 50 psi for the LT).

However, when constantly inflating tires to the max pressure regardless of the vehicle manufacturer recommendations, you can alter the footprint shape which, affects (as you mentioned) the fuel economy, ride, wet traction, dry traction, etc. Which can in some cases also be dangerous due to the adverse affects on safe vehicle handling. Think about it, when you increase the pressure, you reduce the footprint size, less footprint size means less rubber contact with the road, less rubber contact means less friction.

LT245/75R16's are rated to carry 2623 lbs per tire @ 65 psi and 3042 lbs @ 80 psi. Even at 65 psi, you are looking at a max load rating of 2623 X 2 = 5246 lbs for the front axle. Don't quote me on this, but it should be sufficient to carry the front axle load for a loaded E350.

By the way, if you also remember from the Exploder recall, most cases of tread seperation were caused by severe underinflation (below 26 psi) and high speed. Also there was some finger pointing that the tire was manufactured incorrectly. Granted, 26 psi does not seem to be the smartest thing that Ford did as far as recommended pressures, but P-metric tires are built to run at that pressure.