With a turbo 400 & 3.08 gears on a 8.5 rear end and 295-50/15's or 245-60/15's the correct speedometer gear in the transmission should be the 40 "black"one. Only use the 40 to 45 gear drive with that gear. The numbers are on the outside surface of the gear drive housing. It's dead on at 60mph locked on to 3 satellites and the speedometer on the dash.
That's true providing the drive gear on the transmission tail shaft has 17 teeth which yours obviously does. Not all turbo 400s have the same drive gear, though. I just replaced both gears in my T400 to correct the speedometer error. It had a 19 tooth drive and a 44 tooth driven gear which made it right for a 3.08 geared car with a 27 inch tire like a 255/60 x 15. I had to go to a 15 & 40 tooth set to make mine right for my 3.55 gear with the 295/50 x 15 tires.
