If a tire says max pressure 50 pounds cold, what should I be filling these to? The temperature outside is around 35°.
I would fill to 50 psi.
ST Trailer tires are rated to carry the weight posted on the sidewall of the tire at the maximum PSI recommended.
Trailer tires - like auto tires - normally lose air pressure in colder weather.
Always run ST tires at max tire pressure. Always check them when cold. I have found if you have cheap ST tires and rubber valve stems they loose pressure quickly. I check my trailer tires every time, every day I use it. Now that I have better tires with metal valve stems I rarely every have to add air.
They can and will increase in pressure over the course of the day, while in use. That is OK
I believe that the manufacture is instructing you to check the tire before you drive and they are not concerned about the weather temperature. Tires, when ran on the highway, will increase in pressure because of the heat generated by friction from the road surface. A tire at rest will have a lower temperature because of no road friction. Therefore after driving wait for the tire to cool before reading the temperature. Then inflate to the recommended pressure.
You might remember PV = nRT from physics class. Pressure is directly related to absolute temperature assuming negligible volume change. If you put 50 psi in your tires ad 80 F, then the temperature drops to 30 F, you'll only have 45 psi, with no loss of air. It works the other way as well. Tires filled when it's cold out will have higher pressure when it warms up.
There's a secondary reason why the drop when going from hot to cold is usually a bit more than the other way around. Most of the time the air you are putting in your tires is a bit warmer than the ambient because it gets heated by being compressed. PV = nRT again! Actually, it's a little more complicated because the speed of compression matters.
Has nothing to do with the outside temperature. At least that's how it was explained in the tire manual that came with my new trailer. A "cold" tire is one that hasn't been driven on, or something to that extent it was explained. I'm too lazy to go look for the manual, I think it's either out in the trailer in the barn or locked up in the safe.
Oh how shameful .....
FJ... ;) &
Well I found the info.. but I AM too lazy to scan it and post it tonight. Maybe tomorrow!!
I run my trailer tires and also all my car and truck tires at the Max stated tire pressure listed on the tire.
Some shops now promote the use of Nitrogen in tires.
Taking into account all that has been said since my initial answer, I now wish to state the tire pressure should be: 50 psi.
I used 40 psi for about 30 years, as that is what we used on the farm hay wagon.
Last year, I bought a new set of tires and tried 50 psi.
My gas mileage increased from 12 to 15 mpg.