Originally posted by Swifster
Jeri, the rear end gear you use will also depend on the transmission you use. A 4 speed trans will run at a ratio of 1 to 1, from the input to the output in 4th gear. 5 speed and 6 speed transmissions will have 1 or 2 gears that run at less than 1 to 1 (overdrive).

When you combine the gear ratios for the rear end and transmission, you want to try to have a happy harmony. If you run a 3.00 final drive (rear gear) and a double overdrive 6 speed trans, odds are the engine may be turning a idle (or less), even on a 65 MPH highway. At the same time, running a 4.56 final drive with a non-overdrive transmission, you could be spinning your engine at over 5000 RPM at 55 MPH.

Just a rough guess, but if you go with a 5 speed or 6 speed transmission, I'd look at a 3.42, 3.55 or 3.73 final drive. If you plan on a 4 speed, I'd go between a 3.00 to a 3.25 rear gear.
Tom, That's exactly what I was getting at...I know that they all work together, but I don't really understand all the details of HOW they work together...I was thinking of either rebuilding an original Pontiac 4 speed tranny's because of the "nostalgia" factor...but if I understood what the difference between the old 4 speed and a newer 5 or 6 speed was...I could make a decision based on something other than a feeling. It sounds somewhat complicated. Here's a link to a pontiac gear ratio explanation that I found...is this accurate? http://www.wallaceracing.com/reargear.htm

The truck driver just dropped off the latest and last rustbucket...(for now) lol...it's a 78... lots of "surface rust"...i'm sure i'm going to need to replace most of the panels...they're pretty pitted...but the rest of it doesn't look so horrible...well, except for the engine in the trunk lol...