I have always liked 3.91s and have them in the Cuda. But many of my buddies have late model cars and want to hop on the interstate and run with traffic to a destination. So I converted a 518. WOW! What a difference. 75mph on the hiway and the engine sounds very relaxed. Didn't help the gas mileage much, but wasn't expecting it to.
No way would I ever go back to only 3 gears. The only down side is probably the weight. Man, that thing's heavy.
Same reason for converting to OD myself. Car had a 4.10 and expressway driving was not fun. People did it though.
Car has a 3.54 now with the same OD. Nice being able to keep up with interstate traffic.
