If my calculations are correct, the Cougar fitted with the 4.5 Final Drive would top out around 129MPH ... a 14MPH drop. The higher the Final Drive number, the lower your top speed, but the higher your acceleration rate. This means better 0-60/corner exiting, but slower top end. It places the engine at a higher RPM though, thus consuming more fuel at highway cruise speeds. Cruising at 3,000 RPM's in 5th gear now equates to cruising at 3,000+ RPM ... requiring more gas = lower MPG. Expect much better acceleration, but terrible gas milage.