I have 315/70/17 D load range BFG A/T's on my 99.5' V10 ext. Cab F250.

I'm running them at 40psi right now. Admittedly, I've never owned a vehicle this heavy. I Don't tow or haul anything heavy.

What do you gun think I should be running in my tires.

 

I am curious too. I am running BFG AT 325's on factory 18's and have been getting different answers from different tire shops here. All have been much higher than 40psi though.

 

What does it say on the sticker inside your driver's door on the A pillar?

 

First of all, I am honor bound to tell you that you should have load range E tires on your Superduty.

Your load range D tires may have a load rating equal to, or even higher than a stock 17"-E tire. Lets assume that they do, and they are 3,195#. But the LR-E tire does that at 80 PSI and the LR-D tire does that at 65 PSI. Some quick math can tell you what pressure to run.

Your door pillar has the recommended inflation for the stock 17" tire. Lets say it is 70# out of the tire maximum rating of 80#. You would then be running the tire at 7/8 of maximum, which is a factor of 0.88.

Then some simple math can tell us what pressure to run on a 65 PSI rated tire to carry the same load as the 80 PSI tire. The formula would be 65 * 0.88 = 57.2. Thus, to equal the capacity of the stock tire, run the LR-D tire at 57 PSI.

I'm making a lot of assumptions in the above, so change the values to suit your truck and your tire. Change the values in the formula to fit your equipment. Hopefully you get the idea.

Or you could forget all that math and do the "chalk test". Google it. But personally I don't think the chalk test offers much help with stiff highway or all-terrain tires.

 

Thanks for the info, these tires came on the truck when I bought it used. Truck is new to me in the last couple months. Still figuring stuff out.

 

I chalk tested up to 40psi, that's how I arrived at that pressure.

Im just not familar with heavier vehicles.

I'm going to bump it up to 50psi and see how she rides.

 

I checked some tire specs, and I believe your BFG 315/70/17-D are a 50 PSI maximum tire. Check your sidewall to confirm. If so, I would not run them at 50.

50 * 0.88 = 44 PSI.

If you are running empty all the time, the 40 you were running before might be about right.

What does your door pillar show? What tire size and what inflation?