[WardL]

I've got the Roush 427 SR, TKO 600 w/0.82 OD, 3.46 Rear with 17" wheels on my SPF. It is turing 2,200 RPM at 60 MPH and is absolutely fine. When I cruise at 80 MPH (only in Arizona) turning 3,000 RPM you can really start to hear the engine noise and it seems a bit much. Your 15" wheels will increase the RPM a little. It is funny how human nature makes us want what we don't have. If I had a choice, I would get the 0.64. That is because I feel better cruising down the road at 70 turning lower RPMs than imagine I am on the track racing. The 0.82 OD would be better for higher speed tracking, but the 0.64 would be better for cruising down the freeway at 70.