My 70 Dart came with 3.91 sure grip.
I feel that driving it 70 mph for extended miles is asking a lot of my 340. I have the stock 14" wheels with 245/60-14 tires. My buddy works at NAPA in southern Indiana, and hes got a customer with a 69 roadrunner 383 with 3.91 sure grip that he drives on long hauls. But it's got 15" wheels.
Am I thinking correctly that the 15" wheels are changing things for him enough to allow better highway speeds?