Good eyes, tactical driving (knowing where you are), and a radar detector solve that problem. Hell if not that just pull the "2 MPH slower than the speeding car in front of you" trick, nobody goes below 80 in the left lane(s) around here anyway. :glasses7: I hear in places like Texas it's even worse (or better depending on how you look at it lol). Guess I'm just coming up with excuses though as I am a self-proclaimed speed demon
To the OP yes 3.91s will definitely kill your highway cruising ability it will be just like gregpurcell said, driving around in 2nd gear with your current rear gears. I swapped in a set of 4.10s in my Duster to test it out at the drag strip and it got old real fast; I could no longer cruise at 80 with half my powerband left like I could with 2.94s.
And as far as 75 being too much for an old car... Hogwash! lol if your front suspension is in good shape an old Mopar can cruise on the highway as well as any car, better IMO than a lot of newer cars if you put in stiffer torsion bars and better bushings and modify the setup a bit to get more positive caster at the alignment shop. The comment I made about cruising at 100 for 5 minutes that was in the Duster in my sig... never skipped a beat and felt glued to the road the whole time.
EDIT: Although I did discover a minor driveline vibration most likely either due to worn transmission mount or incorrect driveshaft/pinion angle
