I have a 65 Plymouth Sport Fury 383 with 3.91 gears and it does fine on the highway. I prefer two or four lane roads but the interstate is OK too, you wont be holding at 100 mph for long stretches but cruising at 65-70 isn't bad. I wouldn't want to drive it from Miami to Seattle or anything though, that would be a long trip. I had 3.23 in it before, really prefer the 3.91 but I mostly drive around town and stuff.