Speedometer Calibration Check

Requirements/Materials:

You need a working odometer. You must have stock size tires or rolling diameter must match stock tires. A drill motor with a locking trigger is best. You need a constant RPM. Stop watch.

Setup:

Speedometer takes a square rod. A small screwdriver works best. Beat the plastic handle with a hammer until it falls off. Blade end into the Speedometer and the round shaft into the drill motor. You can connect the drill motor with a drill bit but be careful not to damage the speedometer.

The Test:

1. Start the drill motor.
2. Start the stopwatch and push the odometer reset at the same time.
3. Let run for 6 minutes (0.1 hours)
4. Record the speed. It should remain constant for this to work.
5. After 6 minutes, stop the drill motor.
6. Record odometer reading.

Use this Formula:

Speed (mph) X Time (in hours) = Odometer Reading (miles)

Example: Say you turned at 60-mph for 6-minutes (or 0.1 hours)
60-mph X 0.1-hours = 6.0-miles
If your odometer says anything but 6.0 miles, you have to adjust the needle and repeat the test. You can start with 1 minute if you wish. 6-minutes makes the math easier.

Checking calibration while driving: Less precise method

For this I use my 1984 Owners Manual and write down the "Minimum Upshiftting Points".
1/2 - 15 mph / 2850 rpm
2/3 - 25 mph / 2800 rpm
3/4 - 40 mph / 3050 rpm
4/5 - 48 mph / 2700 rpm
Drive up to each point and compare the two numbers. If you're off, you get to pull the speedometer and recalibrate. Again, you must have the same rolling diameter tires as stock for this to be accurate.  Tires must have the same rolling diameter as the stock tires. If not, gears must be changed in the Speedometer by a gauge pro.  Verify the calibration of your VDO Tachometer with a digital Tachometer before checking Speedometer calibration while driving using this "Minimum Upshiftting Points" procedure.