I found a 5-6 lb difference in cold and hot tires. I went to a friend and had nitrogen put in the tires. Now it is a 1-2 lb difference.
I have nitrogen in my tires (The dealer supposibly put it in, the valve covers are labeled N2) and there's still a 4-5lbs difference between hot and cold.
This morning cold it read 30-31 and hot last night was 35. (The car seems to read about 2lbs low)
On my porsche with regular air, I always have a 4lbs difference.
N2 is better because it leaks less (bigger molecules) and shouldn't have any water in it. In F1 they constantly cycle air through the tires to get the last bit of moisture out. Oxygen can also interact with the rubber.
Found some interesting reading here:
http://www.f1technical.net/articles/1
“Moisture in the air makes setting tyre pressures very difficult,” explains Tetsuro Kobayashi, Bridgestone Motorsport Technical Manager. “If you set the pressure in the pits with a cold tyre, or even one that’s been warmed by a tyre blanket, the pressure will be different when the tyre has been brought right up to full operating temperature. This happens even when we use dried air, but this happens in a more progressive and predictable manner than when there is moisture present. If there was moisture present then there would be different amounts in different tyres depending on when and how they were filled so it would be impossible to predict the pressure change and it would be difficult to engineer the tyres to deliver their maximum performance.”
Whilst Bridgestone uses dried air by default, there are other gases available and teams will often try or make use of these in their pursuit for an advantage. In various series, not only in Formula One, nitrogen, carbon dioxide and other mixtures have been used. The most important consideration for whatever gas is used is stability or a predictable change in density relative to temperature change to enable the optimum tyre pressure to be both attained and maintained. A second factor can also be the reactivity of the gas with rubber, a point where oxygen scores badly. At high temperatures, the oxygen molecules in air can react with the rubber and hence reduce the tyre pressure. It is sometimes also believed that the mass of the gas is taken into account as being important, but the truth is that the differences are marginal compared to the other factors. Recently however, the FIA moved to limit the allowed gases to air, nitrogen and carbon dioxide.
In order to allow the tyre to be filled completely with the specific gas, each rim has two valves, one to let air out, while the other is used to pump it up.
Hans.