question: if i my tire pressure is 30 in a 70 degree heated garage and i venture out and drive in a 30 degree day, what pressure will my tires be? (answer at bottom)

well.............................
The tire pressure recommended in your vehicle's owner's manual or tire information placard is the vehicle's recommended cold tire inflation pressure.

Since air is a gas, it expands when heated and contracts when cooled.

The rule of thumb is for every 10° Fahrenheit change in air temperature, your tire's inflation pressure will change by about 1 psi (up with higher temperatures and down with lower). Also note that this is approximate and not precise. In theory, all pressure will cease at −459.67° on the Fahrenheit scale. If you start at 100 degrees & 30 lbs, and loose 1 per 10 then at -200 you would have 0 pressure. proving -1 per 10 is approximate. the formula i believe is.. P = nRT/V

In most parts of North America, the difference between average summer and winter temperatures is about -50° Fahrenheit...which results in a potential loss of about 5 psi as winter's temperatures set in. And a 5 psi loss is enough to sacrifice handling, traction, and durability!

Additionally, the difference between cold nighttime temperatures and hot daytime temperatures in most parts of the country is about 20° Fahrenheit. This means that after setting tire pressures first thing in the morning, the vehicle's tire pressures will be almost 2 psi higher when measured in the afternoon (if the vehicle was parked in the shade).

If the vehicle is parked in the sun, the sun's radiant heat will artificially and temporarily increase tire pressures.
A tire in the full sun at outside temp of 85 can go up 3 degrees, just sitting motionless. (note, since they are already heated up, rolling temp gain will not be affected as much)

Rolling tires will increase pressure. Air pressure of the tires will increase about 1 psi during every 5 minutes of use for the first 20 minutes of operation. Then the air pressures stabilizes, typically gaining no more than 1 psi of additional pressure during the next 20 minutes. That equates to 4 pounds in 20 minutes and possibly 1 more in the next 20 minutes. (personally i found 3 in 20 and 1 in next 20)

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So........ if your in a heated garage at 70 degrees and you venture out to a 30 degree day,

you will slowly loose 4 pounds (70 minus 30 equals 40 divided by 10 equals 4). At the same time, you will also slowly (assuming you are actually driving) increase pressure up to 3 pounds (but more likely 2 pounds). Also if you are driving on a full sunny day, you will gain about 1 pound of pressure. (since your rolling temp gain will not gain as much in the colder outside air and colder roadway and the sun will not be a bright in the winter).

answer to the question at the beginning. what will my tire pressure be while driving?
geeeesh, i dont know, maybe 1 less pounds than you had when parked in your garage????? I would increase pressure from 30 to 33 in the heated garage while the tires are cooled down. Watch the pressure on the DIC. Adjust before you venture out the next time.