Gear Ratio Calculations

Measuring your ratio: on a non-posi (open) rear, the other wheel spins the opposite way, so turning one wheel by hand gives half the true ratio — a reading of ~1.37 is really ~2.74 (a standard 2.78 Pontiac ratio). To measure correctly, either block one wheel so it can’t spin and count, or better, spin the axle and count driveshaft yoke revolutions. Make sure the marks return exactly — close doesn’t count.

Choosing a ratio: for a mostly-around-town 350/TH400 with occasional highway, 3.23 is a good all-around choice (~2600–2700 rpm at 60 mph); 3.55 is livelier but busier on the highway (~2900–3000 rpm). Going much past 3.23 with your existing differential isn’t possible without changing the case. If you’re doing a full rear-end swap anyway, consider a 2nd-gen 8.5″ 10-bolt — ~1″ wider (you’ll re-cut the spring perches), but stronger, more common, and cheaper than first-gen rears. (FYI: code PX = a 1976–81 3.23 posi.)