My garage would get quite hot in the summer so I opened up the drywalled ceiling and found no vents and no insulation. I added a bunch of soffit vents, a vented cupola to the roof and lots of R38 insulation. That lowered the temperature by a good 10 degrees during the hotter summer afternoons.
Another thing to consider is that a couple hot engine blocks in an enclosed garage increases the garage temp as well. If the garage is well insulated that cuts both ways and also retains the engine block heat longer.