Just to keep in touch I note that in the last two weeks our family 2008 Impala has twice signaled low tires via the dash digital readout. Checking the pressure showed the first case had a pressure of about 14 psi and again today another tire registered 19 psi compared to the recommended 30 psi. I say this in view of the discussion above about tire pressure. It occurred to me that the temperature dropped from mid 70s last week to 40 F when I checked the latest low pressure. Let's convert those temperatures so we can use the Charles-Gay Lussac pressure equation to estimate the pressure change due to seasons.
75 F = 23.89 C = 297.03 Kelvin, approximately 297 K
40 F = 4.44 C = 277.59 Kelvin, approximately 278 K
Temperature difference Difference = 19 K
The Ideal Gas Law is generally good up to about 10 atmospheres (147 psi) so we can use
(P1)(V1)/T1 = (P2)(V2)/T2
and if we assume that the volume of the interior of the tire remains essentially the same we can cancel V1 = V2 and use (P1/T1)=(P2/T2). Thus if the pressure was 28 psi at 75 F we can calculate the pressure at 40 F as P2 = P1(T2/T1) so we find
Pressure at 40 F = (28 psi)(278 K/297 K) = 26.2 F for a difference of about 1.8 psi
The same sort of reasoning applies for the case of tires heating up upon use. Of course in a race situation small corrections are made for air temperature as well as altitude above sea level so in that case a car would be set up right at the time of the race. Now add to that the common experience of slow leakage of tubeless tires on older rims and it is my claim that unless I set the pressure of my tires exactly every day for the ambient conditions I can easily see a variation of +/- 1 psi at least and in particular the tendency without inner tubes will be to go to lower pressure when starting out cold and then an increase in pressure during the run of the trip. Thus I think I will still start out at 28 psi and allow for tubeless tire leakage and compensation for increasing psi during travel. Perhaps a future investment of inner tubes could reduce this variation but for now if I am within a degree F or two the exact psi value will mush out due to temperature variations. In the case of our 2008 Impala I think the larger effect is probably due to old rims because the tires are less than a year old while for my roadster the rims are circa 2005 so the same slow leakage is probably occurring. Even if I get inner tubes for both cars we see that the temperature variation will make an accurate pressure setting no better than to about 2 psi.
Now to another important topic, I have not received any suggestions as to how to reduce the size of the temporary file with pictures I have previous put on this forum. The posting procedure says they will be deleted after a short time but maybe the problem is that I am running out of memory on my laptop? If so I should delete some of the 25 that are on there now but how to do it??
Don Shillady
Retired Scientist/teen rodder