A 65 C10 would most likely have a 3.7
My 65 C10 has a 3.73 rear. I used the common method of jacking both rear wheels off the ground and counting tire rotation and drive shaft rotations, (I randomly decided on 20 total rotations of the DS, but more would be even better).
To do the math: Double the DS rotations and divide by the Tire rotations. Mine was 20 DS rotations for 10.75 tire rotations. So 20 X 2 = 40 and 40/10.75= 3.72
Hot Rod Lincoln once explained it well . . .
You've got three moving parts, the driveshaft, and both rear axles. Whichever one has the most drag will sit still, and the other two will move. If you just jack up one wheel and turn the other one, the spider gears between the axles will alter the speed of the other parts. One turn of the driveshaft will result in the wheel turning at twice its normal speed if both axles were free to turn. Remember this if you encounter wheelspin on one side in mud or snow- - -30 MPH on the speedometer will be 60 MPH at the spinning wheel.
The most accurate way I've found to estimate the rear axle ratio is to roll the vehicle straight forward exactly one turn of the rear wheels, and count the turns of the driveshaft. With both wheels on the ground, it's impossible to get an error induced by the differential gearing.
Jerry
