JoegGT3

I have a 72 911 RS tribute that I bought already done. I have stacks of receipts and the car has a 915 transmission from a 1986 Carrera. Shifts great, but the previous owner had it set up with short gears so it runs higher RPMs than I would like. Probably 5k RPM in 5th gear at 80-90mph. 3rd, 4th, and 5th are all very close in ratio.
My question is how can I correct this so that the car has closer to original gearing? I’d like to have a 140-150mph top speed, which I am thinking is closer to factory top speed for an old RS. In order to accomplish this, do I need to have the transmission rebuild with different gears internally, or change a final drive ratio, or simply swap the transmission for a different 915 with the proper gearing? What is the designation of the gearing i am trying to find? Is the close-ratio set up I have currently desirable to anyone? Thanks!

Bill Verburg

An '86 RoW 915/72 @5k will be doing ~117mph w/ 225/50 x16 tires and have a theoretical top speed of ~159mph@6800

A '73 RS @5k would be doing ~113mph w/ 215/60 x15 tires and have a theoretical top speed of ~154mh@6800

Just changing the cwp in the existing trans is not the way to go, unless the original owner shorted the stock 8:31 to a 7:31 in which case going back to a 8:32 should get you where you want to be.

I don't know if you have an lsd but the last year for the good ZFs was 'the '84 915/67RoW or 915/68 US, the /67 would have an oil cooler, next best would probably be a mg case 915/44 from a '76 Carrera, if you go older than that then you have to deal w/ the speedometer drive which I assume is electric w/ a 915/72,, After than any of the 915/6x from the SC series would also be good