PSI Question

[mr.Hand]

In the winter, the sidewall information for tire pressure reads 32 psi. And you are supposed to check and inflate before driving when the tire warms up.

After inflating to PSI, my tire pressure after driving for 30 miles was like 35-36 PSI. Is this correct, and is it safe?! The ride felt way too bumpy. I just checked my pressure on the way home and all four read 32.5 psi.

Also, could PSI differences less than .5 PSI among the four tires make a big difference in the smoothness and quality of the ride?

[Woodie83]

You should inflate to 32 when stone cold and check it at least every three months. It will change the ride quality substantially, but .5 pound is not going to be noticable. Although higher pressure rides rougher, it gives you better performance and mileage. I recommend 35 psi, many guys run 40.

[Woodie83]

Wait, the SIDEWALL OF THE TIRE says 32psi? That's the lowest I've ever heard of, they're usually 40 to 44. If that's the case then you're stuck at 32, but I think you'd better check.

[metro_roy]

Inflate to your pressure when cold and don't worry about the increase when they warm up. See:

http://www.tirerack.com/tires/tiretech/tirespecskey.jsp

Most Geo owners trying to get maximum mileage run at 40 PSI. Some go to 50. A tire is more likely to blow by being under inflated for the load its carrying than by having too much air.

[Mike_Van]

The PSI you choose to run at is always a compromise b/w handling/traction performance at lower PSI and lower rolling resistance/better MPG at higher PSI.

There is no reason you cannot run at least 85% of the PSI listed on the tire sidewall safely. The other posters telling you to check PSI when cold and not to worry about the PSI rise when the tire is warm are correct, as this is normal.

[mr.Hand]

Originally Posted by Woodie83

Wait, the SIDEWALL OF THE TIRE says 32psi? That's the lowest I've ever heard of, they're usually 40 to 44. If that's the case then you're stuck at 32, but I think you'd better check.

oh sorry, i meant the GVW sticker on the inside of the door, not on the tire sidewall

[Woodie83]

Okay, all is right again. The sticker is the manufacturer's recommendation and includes factors such as ride quality and tire wear patterns. The number on the tire has more to do with the strength of the tire carcass. If you're worried about ride quality, it's 32 psi for you. If mileage is more important, 35 is a good number, maybe as high as 40. I certainly would not go above 40, but there are folks who do and have good results. Extremes either high or low comprimise the tires ability to carry weight and provide traction, the most important characteristics of a tire.

Remember the Ford Explorer/Firestone tire fiasco? One factor that helped cause it was that Ford recommended a low tire pressure to improve ride quality, so low that Firestone had complained that it wasn't safe. Add in the normal tendancy of people to ignore their cars completely and the tire pressure fell so low that the tire overheated and failed. All so that the car would feel better on a test drive. Just like the TV manufacturers maladjusting their sets to look better in the store, never mind that the picture is all wrong when you get it home. I would add that the most important factor in the Ford Explorer case is that you'd have to be a fool to drive a vehicle that will roll over and kill you family if you have a flat tire, but that's exactly what SUV owners choose to do.

Also remember that tire pressure changes the handling characteristics of the car. Back in the 60's when Corvairs and Beetles were flipping over and killing folks it was found that the manufacturers had recommended 30 psi rear and 18 psi front to mask a nasty oversteer tendancy. People put 30 all around, rolled over, and played dead quite convincingly.

Simple answer is it's not a simple question. Less pressure = better ride, more pressure = better mileage, things in general should be done in reasonable quantity.