General air pressure question when inflating tires.

I am inflating tires to 40 psi, but it is only 40 degrees outside and tires are cool (not yet driven today). Max tire pressure is 50. So if it heats up to 70 outside and we take it for a drive, and the internal pressure goes up above 50 due to the heat, is that bad for the tire?

What is the general rule for inflation?

I have always heard to measure when cool, but will "cool" on a warm day and "cool" and a cold day make a difference in tire performance/wear when weather warms up and tires get warm?

 

Air changes with the temperature, it is what it is when it's at a certain temperature.
This is some good information:

http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=73

Since, I use Nitrogen at work, I've been using it in my tires, we use to do it on race cars, and I've been doing it for years.

 

I used to use compressed air in our 10 inch circle track racing tires on pavement and it was a real chore to keep the pressures down and the rear stagger from opening up or closing on me. You had to chase it constantly. I changed over to nitrogen and all of that went away due to the much lower, if any, water content in the nitrogen. The tires stayed much more stable.

 

Hey! Thanks for that link. I was not aware that it is only a 1 psi change per 10 degrees temperature change.

5 psi isn't going to make or break anything. Not at 35-40 psi on new tires.

I was afraid it would go up or down 15-20 degrees.

 

If you were playing on the twisties pretty hard, I'd suggest lowering it to around 35psi just to prevent the inevitable increase in pressure from the added friction on top of temp increase, but for daily driving you should be fine. I ran 40psi in my EXP all the time.

 

Your fine as far as safety goes for the tire. You have to watch the wear on the tires though. It differs with pressure depending on vehicle weight,rim dia.,suspension etc. Keep an eye on the tread wear pattern. If it seems to be wearing a bit too much in the center you know your pressure is a bit high.
Max tire pressure at 50 is fully loaded. Check the weight rating on the tire. These on the truck??

 

I recently bought some new tires "Firesstone Precision Tour" for my Windstar van. The seller/installer inflated them to 40psi. I drove abt 15mins on that setup and the car road much to hard for my taste. I deflated them to arnd 32psi where I have been op tires forever. I have not really looked at the tire specs cuz I bought them this winter and not abt to worry abt it now. I will sacrifice a little thread wear for a reasonable ride. I really don't like these tires - they are noisy.

 

I find once you get over about 36 psi most passenger vehicles get real choppy.
Generally the vehicle just isn't heavy enough for the higher pressure. The door sticker psi is a good place to start for a stock size tire.

 

Cool!

I can cut the pressure down a bit to the 37 or so that it calls for.

 

The tires in my service van are at 70 psi, it's an awful ride....

 

Imagine you need it with all that weight in the back though. We used to work on the full sized Bell vans and they needed those heavy high pressure tires as well. Think they were 85 psi max on those Bell vans if memory serves. Real heavy Michelins.

 

40 psi dang! Hauling a trunkful of white lightning?

 

Why are you running them pumped up that high? Is it just the fronts for the drag strip? Other than that, you probably are just running on the treads' centers.

A good way to see what pressure to run and get full tread contact is to start at a pressure that is too high (like 40, cough, cough ...) and draw chalk lines across the tread and up on the sidewalls a bit. Drive the car around the block, moderately, mostly straight lines, go very easy on the corners. See where the chalk got rubbed off.

Drop the pressure a one or two pounds and repeat until you get even wear of the chalk across the full tread without wearing it off the sidewalls. This will give you an idea what your car wants, and may be different front-to-rear. These cars are nose-heavy.

 

Don't worry about the "Max Tire Pressure" that is cast in the sidewall of the tire. It has nothing to do with the pressure that is supposed to be in the tire for the car they are on. It only denotes how much pressure you "can" put in the tire and still be safe. Sence tires of the same design and size can be used on many different cars. Follow the manufactures recommended tire inflation pressures. 40 psi is way to much for a passenger car. Normally on passenger cars the pressure ranges from about 26 to 32. Mini Vans are from about 30 to 35. Light trucks from 30 to 35. Medium duty trucks with LT tires on them from 35 to 50. Heavy duty trucks with 8 to 10 ply tires on them are about 50 to 90.
Under inflation will cause heat buildup in tires and also cause premature wear on the outsides of the tires. It will also cause some squishy feel in handling.
Over inflating the tires will cause the centers of the tires to wear and also cause a harsh feel from the road. The Auto manufacturers spend millions on testing getting the correct happy medium of tire wear, ride comfort and handling to come up with a recommended tire pressure for each car/truck.
Of course all this goes out the window if you are racing of any sort.
Every day we get people that come in the shop to get air that say their tires "look" low but end up not being low. It is kinda hard for some of them to understand that a lot of radial tires look like that. It is called radial sag and that is the way they are designed. Don't get me wrong it is a good thing to get your tires checked and keep them at the recommended pressure.
When Ford had all the problems with the tire on the Explorers and Firestone took all the hit for faulty tires there was a problem with the manufactures recommended pressure that was at fault, not the tire. The OE pressure was 26 psi on those trucks, way to low for the application. There were thousands of tires of the exact same lot that were put on other vehicles with no problems at all. This is an example of one time that the manufacture got it wrong. But for the most part they do get it right and it should be followed. Thus the mandatory federal requirement for TPMS on all vehicles 1 ton and below. To keep tire care and maintenance on the same level as the rest of the vehicle. Yes.................. TPMS is a PITA for the owners of vehicles and the guys that change tires but it does keep people from letting their tires get under inflated. But that is a whole different story.

 

This is on the wife's convertible mustang. I usually run 35-37 for her. Just wanted to try 40 to see what it feels like. I didn't notice any difference.