Proper Air Pressure

[swathdiver]

Been trying to find the balance between comfort and proper wear and proper load carrying capacity of the truck's new tires. They are BFG KO2s LT265/70/17 Load Range C.

The door placard says 30 psi all around for the original Goodyear Wrangler HPs which are P-Metric tires.
The maximum weight for the Wrangler tire is 2,535 lbs @ 35 psi. At 30 psi this number is 2,348 lbs. But that is too high when mounted on a truck/suv. The standard is to then divide by 1.1 so this puts the tires at 2,135 lbs @ 30 psi giving them a combined carrying capacity of 8,538 pounds. The truck's GVWR is 7,400 lbs so the tires can support a little more than 1,100 pounds extra weight when properly inflated.

Whew!

The BFGs are rated for 2,470 lbs @ 50 psi. Their tire inflation and load chart only specifies ratings in 5 psi increments so I had to subtract the difference and divide by 5 to fill in the missing numbers.

For example, at 50 psi the tires are rated for 2,470 and at 45 psi they are rated for 2,255 pounds. The difference is 215 lbs and divided by 5 that is 43 pounds per psi. Curiously from 40 to 45 psi the factor is 36 pounds and from 35 to 40 it is 37 pounds. Does that make sense and am I figuring this right?

Well, if I am indeed correct, 42 psi provides 2,147 lbs per tire or 8,588 pounds for all four. That's as close as I can figure to the originals. Now tomorrow I plan to check my tread depths across each tire and if I can find the chalk, do the chalk test with the tires set at 42 psi.

I've been running them much lower, just completed a 1,200 mile trip with them at 38 psi and currently they are at 44 psi cold. The ride real nice at 30 psi, 44 is quite bumpy.

Comments or suggestions?

 

[HiHoeSilver]

Been trying to find the balance between comfort and proper wear and proper load carrying capacity of the truck's new tires. They are BFG KO2s LT265/70/17 Load Range C.

The door placard says 30 psi all around for the original Goodyear Wrangler HPs which are P-Metric tires.
The maximum weight for the Wrangler tire is 2,535 lbs @ 35 psi. At 30 psi this number is 2,348 lbs. But that is too high when mounted on a truck/suv. The standard is to then divide by 1.1 so this puts the tires at 2,135 lbs @ 30 psi giving them a combined carrying capacity of 8,538 pounds. The truck's GVWR is 7,900 lbs so the tires can support a little more than 600 pounds extra weight when properly inflated.

Whew!

The BFGs are rated for 2,470 lbs @ 50 psi. Their tire inflation and load chart only specifies ratings in 5 psi increments so I had to subtract the difference and divide by 5 to fill in the missing numbers.

For example, at 50 psi the tires are rated for 2,470 and at 45 psi they are rated for 2,255 pounds. The difference is 215 lbs and divided by 5 that is 43 pounds per psi. Curiously from 40 to 45 psi the factor is 36 pounds and from 35 to 40 it is 37 pounds. Does that make sense and am I figuring this right?

Well, if I am indeed correct, 42 psi provides 2,147 lbs per tire or 8,588 pounds for all four. That's as close as I can figure to the originals. Now tomorrow I plan to check my tread depths across each tire and if I can find the chalk, do the chalk test with the tires set at 42 psi.

I've been running them much lower, just completed a 1,200 mile trip with them at 38 psi and currently they are at 44 psi cold. The ride real nice at 30 psi, 44 is quite bumpy.

Comments or suggestions?

Only thing I world add is that the graph for weight vs pressure is a curve. It's not straight, so instead of dividing the difference in 5 lb increments, try actually graphing 3 or 4 points given, and that will give you the actual measurements at any pressure.

 

[swathdiver]

Great idea Nate. Created a graph in excel, psi on the left from 30 to 60 and weight along the bottom in 5lb increments. Plotted the known PSI/WT numbers and used lines to connect them. The numbers were pretty close to my earlier estimates, though more precise. If the designers planned to give a 1,100 pound tire rating over GVWR then my ideal pressure would be 41-42 psi. 42 psi offers 8,600 pounds of lift and 34 psi just over my GVWR of 7,400 pounds. So 34-42 psi should be the range as long as they wear properly.

Well maybe not! The rear axle's rating is 4,200 lbs so a tire would have to support at least 2,100 pounds and that would mean the minimum for these KO2s could be 41 psi in the rear. Or 48 psi to match the originals capacity over GVWR!

So where would y'all set them? 38? 40? 41? 42?

It's raining and I can't go outside to play!

P.S. - I see a lot of Toyota guys running this same tire and size on their trucks at around 40 psi and they weigh at least 1,000 pounds less.

My brain hurts now.

 

[2011SSVHOE]

I think you probably must go by the sticker, and how the tire sits on the ground. Too low and you will wear the sides. Too high a pressure and you will wear out the middle. remember under inflated tires run hotter, and get worse mileage.

 

[swathdiver]

Howdy Roger, I've been varying the pressure from 30 to 38 and now 44 psi. At 30 psi, which is what is called for on the tire placard on the door, though for P-Metric tires, the load rating of the tires is just 1715 pounds, not enough to even support the front axle's GFAWR or the GVWR.

It's been about 6,000 miles since these were put on and they are down to 13/32 tread depth from 15/32. At that rate, they won't make it to 40,000 miles. People routinely double that.

 

[swathdiver]

On the tire placard of a 2011 2500 4WD Yukon, the 265-70-17 Load Range E tires were to be set at 50 psi front and 60psi rear. A day later, my memory revealed to me that on the way home with the truck, the tires were at 48-50 psi. The tires at the time were BFG Rugged Trails, stock size, Load Range E. Maybe that lady at BFG was right? I want ride and longevity; who doesn't?

Will try again to chalk the tires.