Ordered my '70 Cuda 340 with 4-speed and 3.91 gears. Came with F 70 14 tires. Ran about 3,000 rpm at 60 mph. Cruised the roads in western Kansas for hours at 80 mph (4,000 rpm). Assuming you have the 340, it has an almost perfect bore to stroke ratio and can wind all day. It will not hurt the engine, if it is in good shape. People have become used to modern cars that turn 2,000 rpm at 75, which makes the older cars seem odd. If it is a hobby car that seldom goes on long trips, keep the 3.91s it will be a lot more quicker, but you will have to settle for a 120 mph top end.
