I have a sick T5 that will need removing and rebuilding very soon, I have an opportunity to receive a newly rebuilt T5 for payment for a recent job I completed. The question is my current T5 has a .73 final ratio the new one has a .63 and the rear gear is 308, at 80 MPH the engine turns 2600. I know the .63 will decrease the revs to around 2300 at 80 , do you guys think this is a negitive/positive or makes no difference? The engine is a 215 at around 200 HP the body is a 1980, looking for an openion.
Mark, it's hard to say. A lot depends on your engine and it's torque curve and the rear end you're running. If you're getting any sort of "lugging" now when in 5th it will be worse at the same speed. Most V8s have enough torque to pull that type of gear ok though. I wouldn't be as concerned with the rpms at 80 as I would a 65 to 75 where you would probably do more driving. It's there that you want to be in a rpm range that won't lug down the engine.
What is the rpm drop from 4th to 5th with the .73? It will be greater with the .63. With a 3.08 rear gear I'd be looking for .80 5th gear.
Play with Dan Master's gear-ratio-to-road-speed-at-rpm calculator to see the difference:
http://www.britishv8.org/Articles/chart.xls