Originally Posted by unibus
The pressure on the door post is the correct pressure for the tire on your car. The number on the sidewall is the max pressure for the tire. Tires are supposed to be filled cold, once you start driving the tires will heat up and the pressure will soar exceeding the sidewall specification. You are putting yourself and those who travel with you at risk by doing this. Your tires will also wear poorly or prematurely, the slight mileage increase is not worth the risk.
Just my two cents......
I know this is an old post, but this is complete misinformation - and frankly dangerous advice.
The sidewall pressure on any tire is the maximum COLD tire inflation pressure. The manufacturer has already allowed for heat expansion of the tire.
Here's a quick test for anyone that wants to see how much tire pressure changes under heat. Check your TPMS readout when the vehicle has been parked all night. Then go drive for an hour. A little stop and some cornering will generate maximum heat - not highway driving. Watch the pressure change. If your tires are properly inflated, your pressure will change very little from parked cold to running hot.
The door sticker on your car is a recommended pressure for maximuym ride comfort. It has absolutely nothing to do with tire safety. It has to do with making the car ride as comfortable and luxurios as possible.
In contrast, the sidewall pressure is tested by the manufacturer to be the maximum recommended COLD inflation pressure. And rest assured, they put plenty of room for safety factor in that recommendation. All tires have been tested to show that they should be able to run for the full life of the tire at that pressure.
The higher your tire pressure (within the tire manufacturer's limit), the less rolling resistance it has - increasing fuel economy. It also reduces sidewall flex, increasing load capacity and cornering stability, also reduces dangerous sidewall roll should the tire corner at high speed. By reducing the sidewall flex, heat build up is significantly reduced. A low pressure tire will heat up significantly higher and faster than a high pressure tire. And running your tires low on air is far more dangerous than running them high. Higher pressures also decrease tread wear and increase the life of the tire.
The only benefit of running your tires at the posted pressure on the door is a comfy ride. You sacrifice tire life, fuel economy, load capacity and possibly even safety by not running at the higher pressure recommended by the manufacturer. Remember the Ford Explorer recall for tire blowouts? The problem was never the tire itself. The primary problem was the recommended inflation set by Ford. Ford paid far more in settlements than Firestone because of it. The tire in question was rated by Firestone for 42psi cold. Ford put a door sticker recommending 26 psi for a comfy ride. This caused too much sidewall flex, tire overheating, and sidewall roll - a very dangerous combination in a top heavy SUV. Both Ford and Firestone issude recalls. Fords was more extensive. And the Ford recall was nothing more than to replace the door sticker with a new one that recommended 34 psi tire pressure and to inflate all tires to that pressure.
IMPORTAN NOTE. When considering the load rating of your tires for hauling or towing, they are only capable of sustaining that load rating at the maximum sidewall inflation pressure. Any reduction in pressure reduces load capacity!
The best inflation pressure for any car is to run as close to the max sidewall pressure as possible without creating an uneven treadwear condition. This may take some experimentation on some cars.
Consider one last thing. When Jeep makes a car, they only know what tires will be put on at the factory, and even then they ofer some optional tires that are nonstandard (although the door sticker will be the same). How do they know that every set of tires you ever install will run best at the same pressure. Answer is, they don't. While they do place a recommended size and load range on the sticker as well, there is no way for Jeep to know the best inflation pressure for your next set of tires. Again, the ratig on the door is nothing more than a ride comfort setting.
And Yes, I so this for a living. I am an ASE certified mechanic with 22 years experience who manages a full service car care center. My store sold over 14,000 car tires last year and will break that number this year. So I have a little bit of time and experience invested in the tire business.