Sorry to drag out an old post, but I found this one while searching for something else and felt I had to chime in. Its not a good idea, I have done it before on a 20 minute (at most) drive home in my old Jeep one time and it only took me 5-10 minutes of open highway driving to feel dizzy and sick enough to pull over and close the window.
I drove a wrangler and just took out the rear window from my soft top, and had the front windows both open (too lazy after work to take the entire top off along with the rear side windows). Without the top, or atleast just removing all the windows opens everything up more to not create a vaccum
I find it crazy, working in the construction field, that there are people who would start their cars in the winter time in the garage with everything closed, I'm going to assume that your garage isn't sealed very well. And to the poster who said that theres no issue of CO because the cars should have functioning CATS, try laying down next to your exhaust and breathe deeply and see how long you will last!