Here is a map of the ground temps through the US. The ground will radiate heat and the temps are consistent throughout the year as well.

 

Thanks so much. I feel a science experiment coming on. Time to measure variance of temp in the garage relative to outside temperature and the slab over the winter.

 

If you are worried about the accuracy of your car ambient thermometer, get a non-digital thermometer and put it in the garage, a few feet above the floor, and compare the measurements of car vs second thermometer over time and over a range of temperatures. If you are really concerned, make sure the second thermometer is accurate or traceable to NIST.
I've also found that the car thermometer is more accurate when the car is moving with airflow.

 

Or compare:

1) when you pass a bank

2) hear the temperature on the radio

3) compare to Weather Channel app on smart phone

 

You are kidding, right?

 

Not worried about it at all. Just sort of fascinated by how it is possible for an unheated space to be warmer than the ambient temp in cold weather.

Today I learned about thermal inertia and the fact that if you dig down three and a half feet, it's 50 degrees approx (permafrost zones excepted)

 

My assumption is that the car readout is in fact accurate. Thus the curiousity about how an unheated space could be warmer than the surrounding air.
I think Jimbos comments explain it all. Now I am going to mess around with temperature readings to discover just how cold my garage can get!

 

Kidding about what? How to check the accuracy of the car's temp read out?

 

Good deal. You can easily rule out the car heat transfer as the heat stored in the car won't last until the morning to the temperature you stated. A physical property known as specific heat is only high for the coolant. So it stores a lot more heat than oil and the metal.

If you have a deep foundation you could have found your culprit. Concrete has a healthy specific heat as well.

 

Yes

 

Then I'm not kidding. How do you do it?

 

The radiation from the Porsche unobtainium is warming the garage. If you only had Toyotas in there, it would be much colder.

 

With a second thermometer that I know is accurate.

 

I find, banks, radios, and the app on my iPhone to all be really-really accurate.

 

How do you know this?