Alright, 2001 F-150 with 17x7.5 rims. I had a set of ProComp AT's in 33x12.50R17 put on last week and I can not get them to wear evenly across the entire width of the tread. This is an ext. cab 4x4 with V8 and I'm running 36 front (hot), 28 rear (hot). I'm still not getting a full patch, but I'm within 3/4" in from the outer edge of the tread. The tread width is really "round", meaning the contact area of the tread is actually round rather than flat, even with the sidewalls a little buldged like they are. What in the hell should I do with these things? I hear other guys running them on stock rims at 40 psi. all around. At 38 mine don't even contact the outer row of tread at all. These are blems, but that is suposed to only be the visual state of the lettering, which really isn't even that bad. Any advice? Should they not give a full patch? HEEEELLLP!!!!

-AR

 

Without actually seeing the tire it does sound like you are over inflated. However 36psi should not be that drastic. I suspect that your problem is that the 7.5" rim size is pulling your sidewall so far in that the 12.5" tire is rounding over from the small rim. This is just a guess, but the 7.5" rim is small.

Try airing down till you get the contact you want, check the air pressure and see what you have. You can do this with some chalk on a cement or paved drive. Use the chalk to make sure you have contact.

What do the tires say on the sidewall ? Tire pressure recommendation, or whell size ?

-jeff b.

 

Thanks for the help man. I believe this is probably what's going on as well. Too bad nobody mentioned this before I went with them. I researched the matter for 2 months before deciding to go with these. The sidewall says 50 max. door sticker says 35 all around. Right now I'm running 34 front (warm) 26 rear (warm). They really aren't THAT buldged yet, especially in the rear, but they definately are not as inflated as I'm use to. I hear larger tires require less pressure though. There has to be a point where airing down becomes a bad thing. I'm not sure what to do. Guess I'll see what this pressure brings me. On stock rims, people should really stick with the BFG AT 285/70R17's. There actually taller and an inch narrower. Perfect fit I'd imagine. A guy over at the other forum found them on etires.com for $199.

-AR

 

Jackal,

I am sure you have seen this, but if you go to

www.procomptires.com/tirecharthome.html

and then "click" on "MT/AT Tire Sizes" you will see that the tire chart indicates that your tire is specified for a "wheel width" of 8.0 - 10.0 , therefore it appears that your wheel width of 7.5 is too narrow

Is it too late to take them back?

 

I saw that, but everyone said they were fine on their stock rims. There are a lot of guys running them over on FordTruckWorld.com. Guess I'll stick with em'. They seem great except for this issue. Figures, but that's the cost of saving almost 50% on all other tires. Thanks!!!

-AR

 

Ya, that wheel bit makes sense, but just a little tid bit on why you gotta run lower pressure on a bigger tire like that.

PSI (pounds per square inch) means that this is the force exerted on the ground by the tire per given area. The higher the pressure, the larger the force exerted, allowing you to support a larger load. Likewise, a larger area of exertion increases the load capacity. A tire will "flatten" until the surface area x pressure = the load on it. A tire with the same pressure doesn't simply "buldge" (sidewalls) more under a greater load, but rather it "flattens" to increase the surface area that the tire contacts the ground, increasing its load carrying capabilities.

So, for example, a tire inflated to 35 psi and under a load of 1500 lbs will cover 43 sq in. Sounds like a lot, but actually, this is .3 sq ft, or a 3.44" deep accross a 12.5" wide tire, but 4.3" deep across a 10" wide tire. As you can see, the 10" wide tire will "flatten" more to support the same load.

A bigger tire will tend to "flatten" less at the same pressure because its surface area in contact with the ground is inherantly larger. You must therefore reduce tire pressure to increase SA so that the Pressure x SA = load.

 

Huh?

Just an update, I found that 34 in front and 26 in rear while cool, is pretty good. I'm getting pretty close to even wear in front, and the rear is still shy about an inch, but it's close. I figure it's because the rears don't do any steering. Guess that'll correct itself as I rotate them. Could stand to drop another 2 psi. all around, but they'd be so buldged, I'd probably lose 2 mpg., and be shorter overall than I was with the stockers.

-AR

 

Sorry that was the mechanical engineer poking out in me. It just takes less air dammit.

I'm glad you've found something that looks like it will work.

 

I run 30 in the front and 28 in the back and they seem to be wearing good. 20,000 miles since these tires were installed and they are wearing very evenly (and very slowly). I have 35/12.5/17 Procomp MTs

kev

 

I am shocked that you guys run different pressures front to back and even more shocked that you run your tires so low after all the press about under inflated tires and tread seperation.

low in the back and higher in the front is old school and is dangerous and greatly compromises ride.

plus under inflated tire reduce gas mileage, creates heat and causes premature wear especially to the outer edge of the tire.

never ever follow what the side wall says it is just a maximum pressure, always follow the sticker. (Unless of course you have a very different wheel tire set up like really low profile)

no flames just my .02


it does sound like your rims are too narrow for your tires and no amount tire pressure combinations will fix it.

 

No underinflation here, they aren't all that buldged and I'm not even running on the outer edge of the treads, let alone the sidewalls. How is running less in the rear dangerous? There's no weight in the rear, so they don't need as much pressure as the fronts with all the weight on them. If I were to do 35 / 35, I'd probably wear the centers out in 20,000 mi. I believe it's more the tire design than the rim width, as these are made for 8" wide rims, and 1/2" really isn't all that much. I've seen many car tires buldged off rims far more than mine are.

-AR

 

Well, you have to consider the fact that you might have a different load range tire from stock too. They put PASSENGER rated tires on these things stock. That's what the door jamb sticker is refering to. The P235/70/16's that came stock on my truck were a joke. Might as well have been on a Caddy! Forget about hauling or towing anything with those soft tires.

I've got LT265/75/16's now with a load range of "C". It says 50 psi max cold for max load. I usually keep em at 42 psi in the rear and 45 psi up front when empty. When I tow my TT, I'll pump em all up to 50.

I want the fronts with more pressure because they carry more weight when empty. That motor is a pretty heavy thing...... The rear, when empty does not carry all that much weight.

This is just the pressures I've found to be the most comfortable when empty. Besides, if you do run too low of pressure, the tire will heat up more and cause more problems then just wearing more in the middle........ I've got about 30k miles on my current set of tires and they are not even 50% worn. Even the centers. I just rotate every 5k miles and that keeps em even enough.

If you have the stock tires, then follow the door jamb sticker pressures. If they are LT, then you'll have to find the pressure that works best for you.

 

It's the buldging of the sidewalls that causes the heat isn't it? If so I should be alright.

 

The point I was making is that the engineers who designed these trucks feel that for best safety and ride that the front and rear should have the same pressure in the tires. Not all of these trucks came with the little car type tires (Like mine did)

Trust me there is plenty of weight on the back tires at all times. What's 35% of 5000 lbs?

If the door sticker (On any vehicle) states that the front and the rear should have the same COLD psi than that is what you should run. Not necessarily the pressure that the sticker says say if you get a bigger tire or low profile tires you should run what pressure you feel comfortable with, but make it the same all around.

again just my .02.

Be safe-

 

I just know that my door jamb sticker says 29 psi front and 32 rear. Kinda backwards in my mind, but that's what it says.

For the one week that I had the P rated tires, I ran 'em at the listed pressures, and it handled like crap. The front end was so musshy that I felt I had zero control cornering. The sidewalls were actually rubbing the ground on corners....... Looked like the tire was flat too. Sidewalls bulging out. They got hot too. Much hotter then my rear tires ever get, even loaded......

I pumped them up to 35, which was the max cold psi for those tires and it made it a bit better, but were still worthless tires for a truck.

Well, 35% of 5k is #1750. That means that you still have #3250 up front Lots of weight in my book. I want more pressure where it has more load.

It's still all personal preference. As long as you don't overload the tires or vehicle and run them at the tire manufactures specs, you'll be fine. JMO, but 29psi up front for a P rated tire is just to make the 'truck' ride like a Caddy. Great for grocery store hauling and picking up the kids from soccer practice, but don't expect to be able to load that baby up to it's GVWR!

All JMO, and it works for me. Your pressures and comfort level may vary..