[RickeyD]

Garage door vents are unnecessary. Use the garage window w/screen intact, limit it's opening to 8" (very easy to do), construct a cheap wood frame with screening to replace the attic access and thermodynamics will do the rest. Garage temperature will never exceed 5 degrees above ambient at peak during the day.
I have just installed a cheap 20" box fan ( weight about 6 lbs.) and mounted it to my attic access screen. ( negative side of fan towards garage space) About 10AM or so I switch it on and so far the garage temperature has not exceeded the ambient temperature at any point of the day. Also, the attic space temperature above the garage has decreased by 40 degrees. I haven't measured the rest of the attic but I'm sure the immediate area space surrounding the garage attic space has decreased as well.
Cheap fix, awesome result.