[Mustang92]

Just guessing here by I'd say you would be right around 3200 RPM at 80MPH with 4.56s, since I'm at 3050 - 3100 with 4.30 gears in my car. I drive mine in city traffic with no problems but really what it comes down to is how tolerable you are to shifting.

For the differential it really depends on how much track use your car will see. The 31 spline ford unit is a step up from the stock 28 spline unit but not really worth it. Auburns should be burned when it comes to any drag racins. True tracs I've heard are pretty good but not unbreakable. C locker or detroit lockers I've heard are very solid but make lots of noise and can be tricky to drive with. So as you can see there is no easy choice. If you were to stick with DRs and maybe slicks once in a while the True trac would probably be the best bet. For gears 4.56s are the perfect gear for a NA Cobra.

Bill