56 rear axle ratio?
 

Anyone know the standard axle ratio for a 56 StarCheif 2door htp w/V-8,4sp Auto? Trying to calibrate the speedo after 389/350 swap without dropping out diff...Thanks for your help

Hey guys,I found this info on Mr.Gordons web site.(pontiacsafari.com) Says 3:23 for Star Cheif w/auto.I feel this is probably correct.Guess I jumped the gun on this post. Thanks for your time

3.23:1 is likely, but it is easy to compute rear end ratio (assuming no posi-track which I don't think was available in 56) by:

1. putting ONE side of the rear axle on a jack stand to get ONE rear tire off the ground
2. put a chalk mark at the bottom of that tire
3. put a chalk mark at the bottom of the driveshaft
4. put the transmission in neutral and release the parking brake
5. have someone turn the marked tire exactly 1/2 rotation while you count rotations of the driveshaft
6. the number of driveshaft rotations is your rear end ratio.

If it is just over 3 revolutions, it'll be a 3.08
If it is about 3 1/4 revolutions, it'll be a 3.23
Just under 3 revolutions is likely a 2.87

2 rotations of the tire are needed.

I wouldn't bet my life on it, but I'm pretty sure that if one tire is held stationary (on the ground) the drive shaft will turn 2 times the gear ratio for every 1 turn of the wheel. So a half turn of the wheel should = gear ratio number of turns of the driveshaft. I've been wrong (many times today), but I think I'm right about this.

Both tires off the ground for posi rears (in good working order) and one revolution of the tire.

One tire off the ground and two revolutions of the tire for open rears, because of the 2:1 ratio of the spider gears inside the differential.

Try it and you'll see my method is correct. :)

Ever notice how fast the passenger side tire spins on an open rear end when the driver's side tire is sitting still?

That's because of the 2:1 ratio of those fragile little gears inside the differential.

On a posi rear both tires would be spinning the same speed, on an open rear the one that's spinning is going twice as fast compared to a posi rear.

I always turn the driveshaft with both wheels off ground and making sure there is no brake drag . Count turns of driveshaft to 1 revolution of tire. Works fine most times but it is possible to get a number thats way off on an open diff.

That's why you leave one on the ground on a one-legger, for accuracy.

Two turns of the tire, works every time.

I must have had a bad case of oldtimer's disease yesterday. B-man is correct. It is 2 turns of one tire (and no turns of the other), not 1/2 turn like I said above.