The thing about the basement, at least if it is mostly underground is the fact the heated space above radiates to the basement and keeps it warm. There is nothing to radiate to a garage on the side of the house.
In the built environment, the major heat loss is through the windows. The next is through the walls. Third is through the roof on newer homes because that's where the most insulation is. Old houses may be a different story. The degree of loss depends on the thickness of the walls and the R value of the insulation within them.
Any house loses heat through all 4 walls and the ceiling (roof). The attached garage merely takes advantage of that loss by capturing some of it. That's why an attached garage is always higher temperature than the outside (assuming it was insulated as well as the house).
An enclosed basement retains more heat than the garage, because its underground, which is even better insulation. It also captures more of the heat loss from the house because the first floor floor is usually not insulated. Once you open the basement to an exterior door though, you increase the amount of heat loss, plus take in more moisture from the outside. Damp basements are not fun!
In my case, if the house is set at 70F, the basement equilibrates to about 66 with no heat ducts open. If the outside is 25, the garage levels out at about 34. For the garage to get as low as about 25, the outside temp has to drop to below zero.
I work on my boats and cars in the garage year round. If its 35 out there on a day I have off, a small electric space heater warms the area up just fine. Actually, 35 is about the best temp for waxing.....that's when I get the longest lasting job.
