Re: Identify my rear end...
John, your memory is correct. The speed of a driven tire (except with an off-road locked axle) is two times the indicated speed less the speed of the opposite tire.
If you are making a tight turn in a parking lot, the outer rear wheel is making a 40-foot circle and the axle length is seven feet, then the inner rear wheel is making a 33-foot circle in the same amount of time. If the indicated speed is 5 mph, then the outer wheel is going 5.48 mph and the inner wheel is going 4.52 mph. 5 mph time two is 10 mph. 10 minus 5.48 is 4.52.
If one tire is spinning on ice, the other is standing still, and the indicated speed is 35, then the tire on ice is spinning at 70. This is why they heat up and occasionally explode on the idiots who insist on just sitting and spinning when they are stuck.
You can see the same thing if both wheels are raised and the driveshaft doesn't move. The opposite tire turns in the reverse direction at the same speed you rotate the near tire. Zero minus 1 equals minus 1, zero minus 2 equals minus 2, etc.
So, if 4.10 revolutions of the driveshaft will rotate the tire once going in a straight line, it will turn the tire twice when the opposite wheel is stopped. A 4.10 rear axle will rotate the driveshaft only 2.05 times per tire revolution when turning an unblocked tire by hand.
To get more precision, count multiple rotations of the tire and divide the results. Five rotations of the tire with a 4.10 would show 10.25 axle rotations. The 10.25 divided by 5 tire turns times 2 for the other tire turning zero is 4.10. The difference between decimal point variations of the axle ratios will become more apparent the more turns you count, and will offset the difficulty in determining fractional rotation positions of the axle.
Someone said "Making good decisions comes from experience, experience comes from bad decisions." I say there are three kinds of people: those who learn from their mistakes, those who learn from the mistakes of others, and those who never learn.