3.91 good or bad

3.55s would be better, even a 3.73 is pushing it unless you have 28" + tires on a street car for cruising. I had 4.30 and like Joe said you are looking at darn near 4000 rpms at the highway. Good for a reving 1/4 or 1/8 car, or a 3/4 ton truck with big tires etc...

Look for 3.55s