So at the last autox it was cold about 50F (it's cold by CA standards ).

One of my autox "mentors" suggested I lower my tire pressure when the weather is cold and the surface is cold like that. I lowered front by 1 psi and rear by 3 psi from my normal (36,40).

he was right for the most part. but why? I also noticed that after the first run the pressure went up significantly. (37 front and 40 rear, from 35 front and 37 rear).

usually at the other place I run which is in the valley the air temp is 90-95F and the tire pressure only goes up by 1psi after the first run.

 

Couple reasons why.

First off, in colder temps, the rubber isn't as soft, so by lowering the tire pressures, you allow the tire to heat up quicker, regaining that softness. Also, as you noticed, when the tire heats up quicker, the air pressures rise faster than you're used to, and may end up being higher than you normally run in hot weather. This isn't an entirely bad thing, but I suggest resetting your pressures to what works for your car on normal days after a run or two that felt good. In other words, set your final tire pressures "Hot".

The reason for this is that you've determined what pressures work well for your tires when they're up to full operating temperature. The temperatures across your tire might change, but the shape of the tire at that pressure won't. By setting your tire pressures hot, the idea is that the pressure will be correct once you get the tire's temperature up where it needs to be. On cold days your "cold" pressures will be a lot lower than what you're used to, but in the end, your "hot" pressures don't usually differ much.

 

Thanks for the info. I did notice that my hot temps were about the same

 

That was a good point: I believe ur tire will reach about the same HOT temp no matter what the air temp is (unless extreame). So if u do set ur pressure in the morning before u go to autox it will dramatically increase after a run or so.