View Full Version : Rear Axle: Avanti rear axle ratio

09-24-2017, 05:27 PM
6736367362I've tried to search for this info to no avail. I've looked in the shop manual and the parts manual and through the search function here.

My late-63 Avanti has two tags on the rear axle. One states that it is a twin traction axle and to service and lubricate accordingly. The other is stamped "45-C". All that means is that it is a Dana twin traction axle. No indication of the ratio.

The Production order from the Museum only lists "50 Twin traction" as an option.

The previous owner told me that it is a 3.73 axle. How do I go about confirming that? The car also has a 4-speed, but I don't know if that makes a difference or not

The first image is completely upside down, and the second should be rotated 90 degrees clockwise to be viewed properly, but after fighting to get ANY photo to attach in the last 45 minutes, I'll settle for this. This is STILL the most difficult site I go to as far as photo attachment goes.

Thanks. Tom

09-24-2017, 05:38 PM
Since it is a TT, lift both rear wheels off the ground; mark a line on the ground, dead center, below one rear tire, then line up the valve stem at 6 o'clock, with the marked line. Next, lay beside the car so you can easily see the rear driveshaft's rear universal joint; slowly rotate that tire by hand, while counting the universal joint, "flats" (one flat=1/4 turn). When the tire has rotated exactly one revolution, and the valve stem is re-centered on the marked line, divide the flats by four. For example, if a 3.73, the driveshaft will have rotated 15 flats, near as the eye can tell; for a 3.54, about 14 flats, and for a 3.31, about 13 flats, etc..

09-24-2017, 06:40 PM
Thanks Joe

I suspected that both tires had to be off the ground. I have checked other rear axle ratios where the rear was not TT and only one tire had to be free to rotate. Thanks for confirming.

I'm pretty sure that the previous owner was correct in the ratio (he's well known in the Studebaker community), but after 10 years of ownership I thought I'd check.


09-24-2017, 08:12 PM
If you wish, you can take the rear cover off, and the ring gear usually has stamped the number of teeth it has, as well as the number of teeth of the pinion. These two numbers are stamped on the circumference of the ring gear. Divide the larger number by the smaller. This is typical for Dana gears.

Mike Van Veghten
09-25-2017, 12:12 AM
Tom -

IF...the differential is in good condition, what Joe said will work. Both tires should rotate the same direction.
BUT...if the posi unit is worn (worn out clutch plates), it MAY act as an open differential like mine does, and the rotated tire will rotate the opposite direction from the opposite side and tell you nothing about the ratio..! OR, the opposite tire may not rotate at all, or even rotate at a slower speed.
BOTH tires need to rotate the same direction..."equally" (the same speed) to tell you the ratio.


P.s. - What Grobb says is the most accurate, positive way of telling you the exact ratio.

09-25-2017, 07:42 AM
I shoulda added, put the car in neutral and make sure the hand brake is off. Also, if it is conventional drive, leave one wheel on the ground, and count 2 flats as one turn of the drive shaft, i.e. a 3.73 will turn 7.5 flats per one rev of the rear wheel. I have never had to remove a cover yet to determine ratio, TT nor conventional.