What pressure do you keep your tires at?

 

I haven't noticed a big difference in MPG going from recommended to MAX PSI. I did however notice an improvement in handling and braking at MAX PSI (especially in wet conditions).

 

Tom and Ray are not big on the nitrogen-in-the-tires thing:

http://www.cartalk.com/content/colum...bruary/02.html

I agree -- it's a ripoff.

 

On the whole nitrogen thing, I'm a chemical engineer. The ideal gas law equation is PV=nRT, where P=pressure, V=volume, n=number of moles of gas (roughly speaking the number of molecules of gas in the tire), R is a universal constant, and T is the absolute temperature.

In a tire, the volume does not change appreciably, and of course the number of molecules won't unless you add air or it leaks out. There is nothing in this equation that varies with the composition of the gas inside. The idea of nitrogen being better than air for pressure regulation is a hoax unless you want to split hairs.

The only appreciable difference would come from the nitrogen being dry, as any water trapped inside would turn to liquid at lower temperatures. Since it's no longer a gas at that point it will cause a pressure drop. You could achieve the exact same effect by using dry air, or simply filling the tires on a day with low humidity. The change due to water condensation in the tire will be negligible for most of us who don't live in humid climates. Around room temperature, only two percent of the pressure would be from water vapor at 100% humidity! As long as the pressure is topped off on a cold day, it won't drop suddenly later on from water condensing in the tire. It might go up if the temperature warms though.

 

I agree that higher pressure is great for MPG, I ran 45 psi all Summer.

That is not a great idea in snowy conditions however. It is much safer to lower the pressure before driving in snowy slippery conditions.

~John

 

Very nice drcool

 

It is nice to have people like drcool join the group that know what their talking about.

GaryG

 

I put my FEH in service on 10-17-07 and haven't check anything till today(935 miles). I checked to oil and it was just above full. Then I check the tires, all were at 30 or 31. I aired them all to 36. Here in Virginia has been cold, untill last 2 day, so I don't know if they were low from factory or if the temperature change caused the loss.

 

Very Hot ( summer ) to very cold ( winter ) can reduce pressure by 4-5 psi. Also ( as I learned ) filling up in Denver, and driving to Los Angeles can also reduce pressure by 3 psi.

The "pressure" reading on the gauge is the difference between the inside of the tire, and the outside of the tire. Not the actual amount of air in the tire. So 35 psi in Denver will become 32 psi in Los Angeles.
-J

 

I was thinking that my TP could be contributing to my cold weather low MPG, so I checked them. They were all around 40, so I put them back up to 42 and hasn't made a noticeable difference so far. . The winter came early this year, and I really hope it leaves soon!