Stock rear

Anyone know the stock gearing for the 66 GTO rear end? It's a posi, but that's about all I know.

This may help you. If not try searching more on this site
[www.wallaceracing.com]

Awesome, thanks.

Pontiac offered eight ratios in 1966; from 2.56 to 4.33. The standard ratio was a 3.21 for 335 HP engine and 3.55 for the 360 HP. Mine had a 3.55:1 posi (tripower, 4-speed, 1966 convertible).

Careful with the first-to-second full-power powershifts, though! Pontiac uses a two-piece posi case and the 389 produces so much torque that you'll shear the bolts holding the posi case together (I did). I ended up putting a 1968 Olds 4-4-2 rear end under it. It fit with only slight welding to the spring seats. Olds uses a one-piece posi case and if you get one with a sway bar, the handling is flat as a Corvette.

You could always figure out your ratio. Put a chalk mark on a rear tire and on the driveshaft. Jack the back wheels off the ground and turn the tire EXACTLY ten turns while counting the turns of the driveshaft (since you have posi you don't have to worry about what the other rear tire is doing). Divide the driveshaft turns by ten. On a 3.55 you'll see 35.5 turns of the driveshaft; 39 turns for a 3.90 etc.

Good luck and don't let yours get stolen like mine did.