1968 Corvette Rear Axle Ratio

[Joseph M.]

Hello,

I'm trying to determine what axle ratio I have in my matching # 1968 Corvette 427/390HP 4-Speed car. With P255/60/R15 tires on the ralley rims, the engine tacs 2500 rpm at 55 mph. When I jack up the Corvette and check the ratio using the wheel and drive shaft method, the drive shaft almost turns 3 3/4 turns for one revolution of the wheel. I assume that since it almost rotates 3 3/4 turns that it is 3.70:1 but it seems like it should tac around 2700 rpm at 55 mph with this gear ratio instead of 2500 rpm. Maybe the bigger tires is reducing the rpm (not completely shore). Looking straight up at the bottom of the rear axle unit, I can make out an "A" but I can't figure out what the 2nd letter is

NCRS Pocket Spec Guide:
AZ = 3.55:1
FA = 3.70:1

Thanks,
Joe

[Timothy B.]

Re: 1968 Corvette Rear Axle Ratio

Joe,
Probably is a 3.70 axle, counting the driveshaft method does not lie. Have someone drive a newer car at 55 MPH and pace your car to that and see if your speedo is off a little. I don't know what the RPM should be but I bet there is more info about this than you care to know in the archives. Also, don't run into the car in front of you while watching speedo.

[Joseph M.]

Re: 1968 Corvette Rear Axle Ratio

Timothy,

Thanks for the info! I'll try to keep my distance while testing the speedometer in the next few days. :-)

Thanks!
Joe

[Gerard F.]

Count post mile markers

Joe,

To check your speedometer. If you have an Interstate or state highway (in one county) with nice big post mile markers, take a drive for 10 or 20 miles.

As you pass the first marker, record your trip odometer reading to the nearest .1 mi. Then count out the post mile markers and compare with the reading on the trip odometer. If you do it over 10 miles it is pretty accurate, 20 miles is better. Your speedometer correction is just a straight proportion of the odometer reading vs actually miles travelled by the postmile markers.

In other words, if you pass 10 post mile markers and your odometer reads 9.7 miles, your speedometer is reading slow by 10/9.7, or 1.031, that's 3.1% slow.

I did this for over a 100 miles one time, checking it every 10 miles, to come up with my speedometer correction on my 67 with new tires, at 2.3% slow or a correction factor of 1.023.

Works pretty good, but only if your speedometer needle is in calibration with your odometer.

Jerry Fuccillo
#42179