For torque tube type driveshafts we use an old hand held tach/dwell meter and a variable strobe light. Run the engine rpm up to 2000rpm and measure the rpm of the jacked up rear wheel. Engine rpm divided by wheel rpm equals rear end ratio. With an open rear end you multiply by two or on a limited slip you jack up both wheels and read direct. Example: engine rpm 2000, wheel rpm 300, rear end ratio is 6.66.
