I have a set of Toyo Proxy ST 255/40/20 on a set of 20" X 8.5" Foose Nitrous wheels. The tire says that the correct pressure is 51psi but everytime I take it to the Ford Dealer they drop the pressure to 35psi. I am assuming that the pressure on the tire is correct; does anyone know for sure.

 

The psi on the tire is not the correct inflation pressure for your Mustang. It is the maximum psi for the tire. I have stock Pirelli tires, and the tires also say 51 psi, but the label on the door jamb says the correct inflation pressure is 32 psi.

 

I put 34 psi in my 255/45/18 tires. I know that the pressure on the tire is max, but is it max cold? I ask because on my truck, the pressures on the tire and the door jamb are identical.

 

Tire pressure is always checked and adjusted when cold.

As far as inflation pressures are concerned, you are best off sticking at or close to the recommended tire pressure on the sticker in your doorjamb or in your owner's manual. Using too high a pressure can make the tire crown (the center of the tread will bulge outward farther than the shoulders) causing accelerated wear in the center of your tread; additionally, excessive pressure will reduce your traction and adversely affect handling. Using too low a pressure will cause the tire to ride on the shoulders, accelerating wear in those areas, handling will suffer, the tire carcass may overheat leading to ply seperation/possible blowout and if low enough the bead may unseat from the rim.

 

Dont run them at 51 psi!

35 is fine for everyday driving. 32 psi last I checked was a standard for many tires used on a daily basis on many cars from the factory. I run mine at 34 psi and fill them to 40 psi when stored in the winter. (less side wall fatigue and less chance of a flat spot) max is 50 psi on mine

 

The car manufacturers' recommended tire pressure is just that; a recommendation. But it's a good starting point for you to decide where YOU want it. Ride comfort is typically better at or below the recommended value on your car's info plate in the door jamb (however, fuel mileage may suffer). Handling improves as you increase pressure. At your tires max pressure, I'll bet you could have run over a dime and could tell if it was heads or tails.

Begin with your tires at the recommended pressure (probably 32) and drive for the day. Increase the pressure by 2 psi every day until you get to maybe 36 or 38 psi, noting how the car feels each time. Then, do the reverse. Drop the pressure 2 psi each day until you get down to 30 or 32 psi. You may find 2 settings that you like; 1 for city and 1 for highway. It's all personal preference.

Just remember that your tire pressure will change approximately 1 psi for every 10 degree change in temperature. For those minor changes in pressure you need to make, all you need is a standard bicycle tire pump. For my tires, I find about 8 pumps adds 1 psi to a tire. And always check and adjust your pressure with the tires not driven on for at least 1 hour. Even sunshine on one side of the car will raise the pressure on those tires compared to the others.

AND what the heck is your car back at the dealership ("...everytime I take it to the Ford Dealer..." ) for anyway?

 

I take it tho the dealer to get the oil changed because they gave me two free oil changes when I bought the car. The techs drop my tire pressure to what is on the door but that was put on there when the car had the factory wheels and tires; not the ones that I am running now. Does anyone have the tire and wheel set up that I have? What pressures are you running.

 

This is your first car, isn't it?

 

I have 20's also. And your telling me that you should run your tires on what the door says. Even though the side wall height and size are completely different than what came on it stock? I know you should never drive on max, but I'm almost certain w/ tires that did not come on the car. The #'s on the door can not be the same you should use

 

Nothing less than 40 psi all the way around.
Rather be over inflated than under inflated any day...