Rear axle ratio

[Art Upshaw]

I've recently acquired a '62 340 hp and am trying to determine the rear axle ratio. Going through the NCRS TIM & JG I cannot find a ratio tag or helpful markings. There is a "GM" logo cast into the housing with "C23" just below. Help a new guy ? Thanks, Art.

[Joe L.]

Re: Rear axle ratio

Art-----

The casting marks on the differential housing that you described will not provide any insight as to what's inside. In fact, even if there were the typical axle ratio ID tags still on the unit, that still wouldn't say, for sure, what's inside. A lot of things could have been changed in the 40 years that your car's been around.

Your best bet is to determine the axle ratio empirically. To do this, raise the rear end of the car off of the ground. Mark the pinion flange of the differential unit (the part that the driveshaft connect to) with a piece of chalk or crayon. Then, mark the tire with the chalk or crayon. Have someone turn the tire exactly one revolution while you observe how many times that the pinion flange rotates. The ratio between the 2 is the differential ratio. Since there are only a finite and known number of axle ratios used, you should be able to determine which one you have even though it's difficult to count the exact number of fractional revolutions of the pinion flange.

If you get under the car to count revolutions make ABSOLUTELY CERTAIN that the car is supported on sturdy jackstands. NEVER, EVER get under the car with the car supported only by a jack, regardless of what type.

[Art Upshaw]

Re: Rear axle ratio

Thanks for the quick response, Joe. I ain't about to crawl around under the car. I asked Dale Pearman the same question and received the same answer. I thought he was kidding ! Not really important, since it runs and feels great. Just curious.
By the way, are you a member of the Vetteheads nut society ? Thanks, Art.

[Duke W.]

Re: Rear axle ratio

Take the car for a drive. Lay the speedo exactly on 60 MPH in top gear and read the tach as closely as you can. From that data we can probably compute the axle ratio assuming it has the correct speedometer drive and driven gears for the installed ratio.

[Art Upshaw]

Re: Rear axle ratio

Thanks Duke:

I'll give that a try later in the week and let you know. Art.