Ya know, I never thought about using the Mackinac Bridge, but it is about five miles... There's a rise of 150 or so feet in the middle from either end -- and traction would be something "different" -- metal mesh on the inside two lanes. Put your studded snow tires on the race car and nail it when you hit the metal.
I don't have Google Earth for Macs, but if I did I'd show you "the Seney stretch" -- a portion of Michigan M-28 that runs from Shingleton in the west to Seney in the east -- about 23 miles dead flat and straight enough to call perfectly straight, two lanes and paved. No subdivisions, no cows (it does run along and through a National Wildlife Refuge, so there are a few deer), no silos, not even much in the way of wires. Lotsa state police, of course -- especially when the colleges are just letting out for the summer. Look for it in the Upper Peninsula, of course.