Greetings to All, I recall that the '69 C that my Dad had would turn 1600 rpm in 4th o/d at 55 mph, and 2000 rpm at 70 mph in 4th o/d. I do not know if his tach was accurate, or what axle ratio was fitted. Prhaps C owners on the list can provide thier own rpm and speed and tire data to help Will choose the ratio he needs for his car. Eric in Florida 1974 MGB