Air Pres in Larger Tires?

derekmc1 · #1 08-26-2006, 05:00 PM

I just put some 285/75/16 Micky Thompson MTZ on my stock 16x7" wheels, the shop inflate the tires to 50 psi. The sticker on my door say to inflate to 42 psi for the stock 245/75/16 tires.

What tire pressure should I be running?

I want to make sure I'm running the correct pressure b/c they say the tire should have a 7.5" wide rim. I noticed the little nobbies are worn just starting to wear off in the center portion of the tread but are still there on the otter edges of th tire. Are tires designed to run perfectly flat on the road across the whole width of the tire or is it normal for the edges to be slightly "sloped" or not making as much contact?

Thanks,

Derek

kingfish51 · #2 08-26-2006, 05:41 PM

Need more info. Because they may not be the same load range tires, you may not want to use what is on the door. In general, just because they are larger doesn't mean a lot.
What is the load range of the new tires? Are they LT tire? What is the max pressure shown on the side wall?

Looking at the Mickey Thompson site, and assuming I looked at the right tire as they have multiple MTZ tires that size is a load range D LT tire, with a max of 65psi. I would run in the 45-50 psi range. 42 might be a little low for a LR D.

derekmc1 · #3 08-26-2006, 08:25 PM

Yes the load rating is D. they are LT285/75/16. I am not up to snuff on tire ratings and what they alll mean, doeas a D rating need more poressure?

derekmc1 · #4 08-26-2006, 08:26 PM

Sorry max psi is 65.

kingfish51 · #5 08-26-2006, 08:33 PM

In general because of the higher max pressure and rating, they use a little more pressure, but can go to a higher pressure and handle more weight. Personnally I would try around 45-46 and keep an eye on the wear.
Your old tires, were they load range D? It almost sounds that way from the pressure on the door, or they were P series (passenger car) extra load tires and were near max pressure. If the old tire were load rang D tires, then I would try using what was on the door, as that was Ford recommended for that LR.

Padron · #6 08-26-2006, 09:03 PM

I have my BFG MT's 285/70/17 filled to 55psi, max pressure is 65psi. Seems to be giving very even wear on a 7700lb Expy, I did have them at 50psi for about 3,000 miles but the sidewalls looked a little low / under inflated.

Now I have 10,500 on them and they are wearing great, you may get away with lower pressure and still get even wear on your F150.

Maybe try 50psi for 3,000 miles or so, and look for abnormal wear ( cupping ) raise air pressure if you see any issues.

Spinal · #7 08-26-2006, 09:33 PM

wow thats pretty high, on my truck 275/60R17 they're only 32psi.

wandell · #8 08-26-2006, 10:23 PM

I've been running 40psi in my 285/75-16 BFG AT KO's since 2001. I currently have 51,000 miles on them and they are wearing evenly.

Oxlander · #9 08-26-2006, 11:46 PM

I recently had some Nitto 295/70R17 load range D installed. When the installer aired them up he put in 40psi. but after driving it like that for a few days the ride was too "mushy" for my taste. Now I have them aired up to 55psi and the ride quality is far better. A little bit stiffer and with less body roll in turns. These tires are rated for a max of 65psi so I figure that 55psi is well within the tire's operating range.

Ox

kingfish51 · #10 08-27-2006, 07:19 AM

Quote:

Originally Posted by Spinal

wow thats pretty high, on my truck 275/60R17 they're only 32psi.

Those are passenger car tires. P series.

derekmc1 · #11 08-27-2006, 07:18 PM

What is "cupping"? I was thinking that if the tire was over inflated then the middle would bulge out and wear quicker than the sides is this correct?

I dropped the pressure to 46 psi and noticed that the tires is starting to scuff up the full width of the tread, where as before only the center treads loked marked up and the side lugs looked untouched towards the outside. Is this good the have the tire running on the whole width of the tread? or is it normal for the side lugs to be off the ground a bit?

derekmc1 · #12 08-27-2006, 07:19 PM

My old tires were LT245/75/16 rating D

What is "cupping"? I was thinking that if the tire was over inflated then the middle would bulge out and wear quicker than the sides is this correct?

I dropped the pressure to 46 psi and noticed that the tires is starting to scuff up the full width of the tread, where as before only the center treads loked marked up and the side lugs looked untouched towards the outside. Is this good the have the tire running on the whole width of the tread? or is it normal for the side lugs to be off the ground a bit?

kingfish51 · #13 08-27-2006, 08:12 PM

Yes it is good that the tire is scuffing across the full width. If it only wears in the middle, it is over inflated. If it wears mainly on the sides of the tread, it is under inflated.

Here is a short explanation on tire cupping. Usually caused by out of balance tires.

http://www.partsamerica.com/MaintenanceTireProblem.aspx

Jackal · #14 08-28-2006, 04:02 PM

I am running my 33" ProComp XAT's (load range E) @ 40 lbs. front / 34 lbs. rear. I'm not getting much bulge at all in the sidewall and the whole width of the tread is making contact.

derekmc1 · #15 08-29-2006, 06:02 AM

Jackal,

Whats the theory behind running lower pressure in the back? Is it because the rear is lighter?

Thanks,