Rarlyl8,
I'm hav'n some fun going through my old magazines and just came across a road test of the 1978 911SC in the April 78 issue of R&T They show the same overall final drive ratio as the 85 model but a calculated 23.2 mph per 1000 rpm in top gear(with 225/50-16 tires). They state 60mph should be 2590 rpm. It is possible that your car's final drive is different as they are available.
Seth
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