Dont want to hijack the thread, but TJ what psi are you running in the AT's? I was waiting for you to respond to Gonzo.
You have to realize that there is no one psi number that is RIGHT as far as tire pressure to improve mpg/ride well, etc.
If MPG ALONE is the issue, you COULD inflate to the Max Psi on the sidewall and call it a day.
It would have the lowest rolling resistance, as the tire would not be flexing the rubber much as it rolled, as the sidewalls and tread would be stiff and round, and not deform/reform as much as the tire rolled along.
It would ride very rough/bouncy, and the contact patch would be minimized...hurting traction, braking, handling, etc.
Most tires have a "Sweet Spot", a load/psi balance that they are designed to work best with...a balance between enough stiffness to make it handle well and have a good contact patch, but not have a bulging wishy washy sidewall, etc.
The idea is to have enough pressure to support the load, but not too much more than that...while ALSO having the tire inflated to a high enough PERCENTAGE of its max psi to be within design limitations.
IE: Two tires might have different load ratings, and, different design loads in mind when the carcass and tread, etc, were designed....
...if a heavy CAPACITY tire has a giant sidewall bulge as its running around with only a few PSI, as that's all it NEEDS to support a LIGHT LOAD...it can physically support it easily...
....but, the handling will be very wishy washy due to sidewall sway, and, a lot of HEAT will build-up due to increased hysteresis of the sidewall and tread rubber, as they will be flexing/deforming/reforming like mad as the tire rolls, etc.
So -
Even two sets of
BFG AT ko's might be totally different, depending upon load range, etc.
The off the show room floor/industry standard to achieve the best ride/mpg compromise is to inflate the tires so as to have the psi supporting ~ 35% of the rig's GVWR in each tire.....WITH the tire being inflated to at ~ 65-80% of its max psi.
(The tire will not as work well if essentially running around too far out of that performance range...as the "Sweet Spot is typically ~ 65 - 80% of its Max Load)
So, if my 305/70/16 with a max load of 3,525 lb at 65 psi
were set to support 35% of 5,200 lb (The GVWR), that would be 1,820 lb.
TO support 1,820 lb, which is ~ 52% of my tire's max load
(1,820/3,525 = 0.516 = ~ 52%)
I'd need ~ 52% of my tire's max psi....or about 33.5 psi
(.516 x 65 psi = ~ 33.5)
So - MY 305/70/16's need a MINIMUM of 33.5 psi for an X...
If someone else has ANOTHER set of BFG AT ko's, but the popular 265/75/16's for example:
Those are rated with a max load of 3,415 lb at a max psi of 80.
1,820/3,415 = ~ 53% of the tire's max load
0.53 x 80 psi = ~ 42.5 psi
TO SUPPORT THE SAME LOAD AS THE 305/70/16's which only needed ~ 33.5 psi to do the same thing.
I use
closer to 45% of the GVWR, instead of 35%, as the factory balance is very mushy to me, and heavily biased towards a soft ride and heavy under steer...what Joe Consumer/Corp Lawyers typically prefer.
My rig also weighs about 6k lb when loaded for work as well, another psi increasing factor.
So, I selected the 305/70/16 BFG AT ko because I need to support up to ~ 6,000 lb.
If I substitute the 6k lb for the GVWR of 5200 lb, I get 45% of 6k = 2,700 lb.
2,700 lb/3,525 lb Max Load capacity = ~ 0.76%
0.76 x 65 psi = 49.5 psi
THAT all gets me enough psi to support the load, within the sweet spot of the tire's load range, and within my own personal ride/mpg balance preference.
If my load is less, I run lower psi to compensate, etc.
IE: I might be running around with an average of 40-50 psi, depending upon what I'm doing that day.
I have OBA, so I can adjust at will.
IF I had those 265/75/16's.....
2,700 lb/ 3,415 lb max load = ~ 79% max load...still within the Sweet Spot typically.
0.79 X 80 psi = 63 psi
So, If I have the 305/70/16's, I might run ~ 50 psi, and if I had the 265/75/16's, I'd need to run ~ 63 psi, to get the same result.
And so forth.
Does that make sense?
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