what tire PSI do you guys run on 33x12.5's? I'm trying to get them to be filled up for optimal fuel efficiency and tire wear. I currently have them at 29 psi and they are over inflated.

 

what do you mean they are "over inflated"? the pressure rating should be 50 psi. If you are over 50 pounds, then the tires are over inflated.

edit: excuse me 35psi.

 

your 33s are OVER inflated at 29 psi? if you don't mind my asking, how do you know theyre overinflated at that pressure? you should go with what the tire says, which is usually around 35 psi isnt it? i keep mine around 37 or so. the higher the pressure, USUALLY the better the efficiency (less contact area, less rolling friction) however the more uneven the wear and the higher the risk of exploding while driving. do what the tire says, its not there just for fun

 

maybe it's 39, but when I go on a dusty road you can see that the dirt is not getting to the full side lugs on the tire

 

yep it's 39 and there are many ways to check to make sure tires are properly inflated like putting chalk on tire tread and driving in a straight line. I'm trying to get optimal tire wear and MPG. Just wondering what others have there tires at.

 

I run 35psi in mine.

 

that's one of the big problems with 'wide' tires... the balloon effect... where the center of the tread grows to a greater diameter than the edges (have the same problem with the tires on my firebird, if it makes you feel better).
Radial tires tend to not have the same issue, or at least not as bad, as bias-ply tires, since their reinforcement runs parallel to the tire centerline as opposed to across it.
Regardless, the highest pressure you can run and keep the outer tread blocks on the ground is going to be the best balance between mpg and wear. but you will still get more wear in the center due to centripetal force causing the center of the tread to expand more than the edges of the tread.

 

Many 33x12.50-R15 tires have a 35psi max inflation pressure.

Some tips on picking a good inflation pressure on the web page below:
- http://www.4crawler.com/Diesel/Tires...roperInflation

I find with my 33x10.50 tires on 6" rims, I have to compromise on pressures, about 20 psi gives me a good flat contact patch and I'll run that around town. Then for longer highway trips, I'll inflate up to close to the max pressure (50 psi on the narrower tires) and that will give a little less rolling resistance at the expense of a little more center tread wear. But then, when I air down and take them off-road, the outer tread lugs tend to take the most abuse and wear, so it usually evens out in the end.

 

The wider the tire, the less pressure you need in it. Just as a wider tire exerts less force/area on the ground than a narrow tire, the ground exerts less of a force/ area on the wider tire. Its just basic physics.

My best guess is that you'd calculate you're tire pressure like this.

Stock front tire pressure with 235/75r15: 27psi (I think)
235mm =9.25"

9.25"/12.5"= 0.74

thus 27psi x 0.74 = 20psi

That's not even accounting for the length of the footprint going from a 28" to a 33" tire. If you took that into account:

28"/33"= 0.84

so 20psi x 0.84

=17psi

So, if my theory is correct, 33x12.5s at 17psi will hold the truck up the same as a 235/75r15 at 27psi. You'd best try a chalk line test as the tire constructions may differ a bit between a 9.25 and a 12.5 wide tire.

 

When I was running 33x12.5s on an 8" wheel I was at 32psi, with no "ballooning" and the wore pretty good...I was 16 at the time, so I wasn't really paying attention to mpg gains/loses.

 

matt16... I didn't see anything in your calculations regarding or factoring in the increase in friction incurred by a deformable body traversing a planar surface. you yourself said the trick to not negatively affecting mileage would be to have a contact surface area approximating that of the original tire, and factoring in the added mass of the larger tire. and then you'd also have the gear ratio's to contend with as well as the BSFC of the engine, peak torque and peak horsepower... aerodynamic drag incurred do to the larger frontal area presented by the larger tire....

your calculations appear to have more in common with selecting jounce and rebound rates for shock absorbers than minimizing rolling resistance and increasing mileage.

Don't get mad at me... just pointing out potential flaws in your calculations... maybe something you hadn't considered.

 

Ive always stuckaround 32psi. Ikeep my tires rotated an they always wear even. Not very scientific , but thats what ivefound the best for me

 

No, you are certainly right, the calculation is no where near accurate, just an attempt to demonstrate why a bigger (wider especially wider) tire doesn't need to be at as high a pressure. That's all. The chalk line test would be way more accurate as a means of determining what pressure is correct.

Luckily, radial tires are pretty tolerant of a wide variety of tire pressures. My understanding is that bias ply tires were way pickier, but that was way before my time.

 

Just ask tire shop.I run 34 in mine.

 

just a little tid bit of information-
your truck hangs from the top of the tire...
not supported by the bottom of it.

 

Mud grips spinning in the wind ain't aerodynamic.
Bigblock