Classic/Antique Car Repair: rear end ratio, stoplight drag races, gas hog

Question
I have a 1965 gto that runs too many rpm at 60 mph making the car unpleasant to drive for long distances. Is there a way to turn the tires on the rear axal and count the revolutions of the driveline to figure out what the gear ratio is?

Answer
Somehow, my answer got lost in the system - this happens once in a while.

Anyway, yes, if your car does not have a limitted slip differential, you can jack up one rear wheel, and with the transmission in neutral, count the number of turns the driveshaft makes when you turn the one free wheel two exact turns.  The result will be very close to your axle ratio - as close as you can get it that way.    You can also slither under the car and look for a tag on one of the cover bolts on the differential - if you find something like a sheet metal tag with some stamped numbers on it, that will be the rear end ratio.  It may say something like 3.73, for example, or it may say 11/41 - to get the ratio you divide those two numbers.  11/41 would be a 3.73 (41 divided by 11 is 3.73, very closely).

If your car does have limitted slip, you are not going to be able to turn the one wheel very easily with the other side on the ground, so you'll have to try to find the tab first.  If you can't, you'll have to jack up the whole rear end of the car, and turn both rear wheels the same amount - then the ratio is the number of turns of the driveshaft for ONE revolution of the two rear wheels.

A lot of original GTO owners regretted that they ordered a car with a fast rear end - 4.11s and 4.56s were popular back in those days. If you have one of those, it will be a real gas hog, and not pleasant on the highway - that car was built to win the stoplight drag races!.  You might have to search for a rear end out of a milder Tempest - maybe you could find one with a 3.07 or even a 2.70 or so in the wrecking yard.   Your speedometer will read way too low, though, if you do that, so be careful - you'll be going 100 when you think you are only doing 75.

I hope this is clear enough- if not, ask again,

again, sorry for the delay.

Dick