metroplex

I have a 07 Mustang GT that had stock 235/55R17 tires, and the door sticker said 32 psi. I noticed the 07 Shelby GT with the stock 235/50R18 tires also had 32 psi listed on the sticker. I read that the 07 GT500 with the stock 255/45R18 front and 285/40R18 rear had 35 psi listed on the door sticker. My 2014 SHO with 255/45R19 has 34 psi listed, and the SHO is about 4300 lb (slightly heavier than the 07 GT500).

I went with 255/45R17 tires on the stock wheels for the 07 GT. Should I stick with 32 psi or go to 34-35 psi? I'm thinking the 50 or 55 profile requires less pressure, but the 45 profile requires more pressure.

I just want to make sure I get the most even treadwear and not have the sidewall damaged from our rough roads. There doesn't seem to be a rule of thumb for this.

SpectreH

I'd run the higher pressure. I think you are right, the lower aspect ratio requires more pressure.

NC14GT

34-35 psi is perfectly OK. Even running that pressure with 'stock' size tires won't wear them out either. If you were talking 40 psi, yeah, you would wear out the middles plus have a harsher ride. You will be fine.

metroplex

It looks like the 03 Cobra with stock 275/40R17 has 32 psi on the door sticker, so there doesn't seem to be any rhyme or reason. I think you're right, I might just stick with the door sticker value.

1 Alibi 2

According to TireRack, run the PSI listed on the door sticker, tire size has nothing to do with it.
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metroplex

Out of curiosity, why is that the case? If I go to a lower profile and wider tire, there would be less sidewall and possibly less air volume. Or in some extreme cases, a higher load rating tire might be used.

Jazzman442

Always run the tire pressure on the Door. Unless you are running slicks.

Bert

Theoretically the tire pressure is always the same regardless of tire size because the contact patch is always the same -- pounds / pounds per square inch = square inches

For practical purposes the door jam sticker value is a good starting point but there is nothing magic about it.

Generally you will get a hair better gas mileage with higher pressure due to reduced rolling resistance. Generally you will get better traction/grip with lower pressure, up to a point; at the expense of reduced steering precision.

One way to check for "optimum" pressure is tire wear: if the tires are wearing evenly all the way across the pressure is about right; if they wear more in the middle the pressure is too high; if they wear more toward the edges the pressure is too low.

Another way to check is cornering roll-over, but that really only works on extreme cornering like on a track. Most tires have roll-over indicators on the sidewalls; usually they look like little triangles, on Michelin tires they look like the Michelin tire man. On extreme 99% cornering, if the tire is rolling over to the middle of that mark, the pressue is optimum. If the tire is rolling over past the mark, the pressure is too low; if it is not reaching the mark, the pressure is too high.

metroplex

Wooo I did not know that about the roll-over indicators. I have seen various markers/triangles before and used them to gauge wear, but never knew that was a proper way of doing it.

BTW when Discount Tire installed the new VDO TPMS sensors, they left my factory banded TPMS sensors on the wheels and never removed them.

m05fastbackGT

I run my Nitto NT555 G2's between 32-35 PSI..

houtex

I run my 275/45-18s at 32 PSI +/- .5 PSI cold.

I've found the car will wander a little more if I go to 34 or so. If I go less, the car starts to squeal and not hook up as good in the back.

Happy medium, that 32 PSI. Been doin' that for years now, and it seems to be fairly consistent. No wear issues, and MPG? Whatever, it's a Mustang GT. If I wanted MPG, I'm gonna get a Prius or something.

07 Boss

Generally speaking the narrower the tire on a specific set up the higher the pressure you need to run. You can't compare pressure from the same size tire on different cars. They are all specific to particular car set ups. You can;t even compare pressure on the same cars with different suspensions or power levels. Engineers use tire pressure to help make the cars handle the way they want, to help balance out the car. With that said most cars are built with a bit of under-steer, they are just safer that way.

Anyways, start with the recommended pressure and play with it from there till you get it where it feels good. Personally I run a really high pressure up front, 38-40 and 28 in the back. I think the front are 255/45/18 and the rear are 305/40/18. But my car is a very unbalanced and ill handling ride with an extremely stiff rear and soft front suspension.. It will all be about personal preference.