[GeneBob]

I jacked up the passenger side rear, removed the spark plugs, put the trans in 3rd, turned the engine hand crank and counted the turns of the rear wheel. I turned the engine by hand for several turns to reduce any error in guess-timation of the turns of the rear wheel. I then took the number of turns of the wheel and divided by two (becasue of the differential effect) and took that result and divided the number of turns of the engine crank by that result to get the approximate rear end ratio.
The possible ratios are discrete choices so you get to the correct ratio pretty easily.
10 turns of the crank gives about 5 and 1/4 turns of the rear wheel. 5-1/4 divided by 2 is 2-5/8. 10 divided by 2-5/8 is 3.8 which is close to the 3.78 rear end ratio.
I did this because my tachometer was giving me strange RPM's compared to the chart above. The tach was getting interference from an old condenser in the distributor.