Dumb ? - Tire pressure?

Cool Wheels said:

This is almost embarrassing to ask. What's the correct tire pressure for a 302?

I don't have the door sticker. Never have.

I've always kept my pressure at 29 psi up front and 27 in the rear. That's my best guess.

With the new wheels on, I'm wondering if that's right or close even.

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Seems a little soft to me, unless traction is your biggest concern. I'd use between 30-35 all around.

IIRC 28 to 32.

It's not, I didnt remember coorectly.

http://stangnet.stangii.com/tire_pressures.jpg

The max inflation pressure is printed on the sidewall of the tire.

Max inflation is based on load carrying ability. I ran my 02 GT at what the sidewall said and wore out the center of the tire. When I replaced them I was told to run them at the pressure on the door sticker which is about 5 lbs lower.

jeffnoel said:

Max inflation is based on load carrying ability. I ran my 02 GT at what the sidewall said and wore out the center of the tire. When I replaced them I was told to run them at the pressure on the door sticker which is about 5 lbs lower.

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People were doing that with the Explorers, but after a few blowouts, Ford started suggesting that they go by the sidewall print instead of what was on the doorjam.
How could the tire manufacturer not know how much pressure to run in their tires?

I guess I didn't look closely enough at the original posters question, with my initial response, but I still stand by my claim. It seems he WAS inquiring what was the factory recomended pressure, per the door sticker. My response however, was based upon my preferences, and past experiences with tires in general...and also may not necessarily apply to many of todays super low profile tires.

In any case, I doubt any tire manufacturer would "recommend" inflating tires to the "max" psi which is on the sidewall for everyday driving. And car manufacturers generally have set unrealisticly low pressures for their own reasons, mainly to ensure a soft ride. After all, under nomal driving conditions and using average annual mileage allowances, even somewhat underinflated tires usually lasted through the old warranty periods of 24mo/24000mi or 36/36000. And I believe the warranty of the tires was separate from the vehicle anyway, so the car manufacturer didn't bear the cost of replacement if one was later requested. By that time, most people figured they just needed to get a new set anyway, with something better than the OEM tire....and seldom replaced them through the dealer that sold the car.

For as long as I can remember, 30-35 psi has been generally accepted as a good all around compromise for tire wear, vehicle handling, and fuel mileage considerations. I'd expect anyone running 24 psi, to have subpar steering response, poor mileage, and quicker than average tire wear. Inflation upwards of 35 would generally produce a harsh ride, and less than ideal tread contact with the road.

BTW, it's interesting to note, that the door sticker lists a higher inflation pressure requirement for the 2.8 models, which were also using a smaller tire.
I can't figure the logic here.
Also, keep in mind that these recommendations were from the early 70's, when radial tires were a fairly new and uncommon item on many cars. I remember, that many people were often concerned about the way radial tires always looked underinflated, compared to the old bias plys. I belive (in part) because of this, installers and tire service centers moved to make 30 psi a general standard of inflation, as the radials became more standard equiptment.

Blue Thunder said:

People were doing that with the Explorers, but after a few blowouts, Ford started suggesting that they go by the sidewall print instead of what was on the doorjam.
How could the tire manufacturer not know how much pressure to run in their tires?

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Because the tire manufacturer doesn't know what vehicle the tire may end up on, so the max tire pressure on the sidewall relates to the max load carrying ability of the tire.

Example, if the tire is rated at 1500lbs at max pressure say 32psi but the vehicle only weighs 3400 lbs if your run at max pressure the tire is overinflated.

jeffnoel said:

Because the tire manufacturer doesn't know what vehicle the tire may end up on, so the max tire pressure on the sidewall relates to the max load carrying ability of the tire.

Example, if the tire is rated at 1500lbs at max pressure say 32psi but the vehicle only weighs 3400 lbs if your run at max pressure the tire is overinflated.

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If you squeeze a balloon, the psi inside the balloon will rise. Likewise, if you squeeze a tire by applying more downward force on it, the psi in the tire will rise. Therefore, if you have a light car, and you fill the tires to 30psi, and you load the car with a bunch of people and equipment, then recheck the tire pressure, it will be slightly higher than it was when unloaded.

Also, a softer(lower air pressure) tire will generate more heat than a harder(higher air pressure) tire, which causes the air inside the tire to expand at a greater rate, therefore increasing the pressure in the tire. So if you start out on a cold morning(say 50F) at 28psi in one tire, and 32psi in the other, the 28psi tire will generate more heat in a much shorter time, increasing its psi, and so at operating temperature,(100+F) the two tires will have a more similar pressure than they did when cold. The difference being, the lower psi tire generating more average heat than the higher psi tire.

All that aside, anytime I see a tire rating with 35psi at max load, I run them between 30 and 32, depending on how the car feels.
The ZR 17s on my II have a max sidewall rating @ 45psi, and so I'm running them at 42, and it seems to work well. I'd be nervous running tires softer than 5psi under the max rating.