how much difference between 3.91 and 4.10

mshred said
...does the fourspeed in your dart sport make a difference rpm wise on the highway?

Not if its a standard A-833, the 4th and final gear ratio is still 1:1 just like 3rd gear in a TorqueFlite.

If you went with the latter 75-87 OD A-833, you would basically have a 3 speed with an Overdrive 4th gear. 3rd gear is 1:1 and 4th I believe is 0.73:1 which would make your 3:91 or 4:10's very driveable on the freeway.

Check out this link below, I screw with this calculator a lot and hope it helps you.

http://www.bigblockdart.com/techpages/speedtire.shtml