3.23 gears and speedo cable

There's a pretty big difference between 3.91's and 3.55's. My car had 4.10's in it when I got it. Man they suck for the hwy. At 60 it sounded like the engine was going to come apart. I put in 3.55's and they do great on the hwy at 60. At 70 their humming a little bit but not terrible. We frequently go to a car cruise that's 40 miles down the hwy and run 70 all the way. That first link in Poison Dart's post is my favorite calculator but you have to remember to put in your tire size accurately and if your car is an automatic dial in some torque converter slip. 7-8% slip is a decent converter. I mentioned inputting the tire height because it makes a big difference on cruise rpm also. The factory tires were real short. Stick as tall of a tire as you can get under the wheel well safely and it'll lower the cruise rpm just like gears do.