Hello cardog,

I found the easiest way to check the speedometer error is by using the following formula.

You will need to know;
1. The rear end ratio of your car;
2. The type of gearbox, ie M40 4 speed, M41 w/ d-type overdrive or M41 w/
j-type overdrive;
3. The length of one rear tire revolution (circumference) in inches.

You will also need a tachometer.

I measure one tire revolution in the driveway by making a chalk mark on the bottom of the tire and the driveway and then rolling the car forward one tire revolution to where the tire chalk mark is at the bottom again. Then measure the distance between the two marks in inches.

The formula:

w/ M40 (4 speed):

Miles per hour = engine rpm X tire circumference (inches) X 0.0009470 / rear end ratio

w/ M41 d-type :

Miles per hour = engine rpm X tire circumference (inches) X 0.001257 / rear end ratio

w/ M41 j-type:

Miles per hour = engine rpm X tire circumference (inches) X 0.001188 / rear end ratio

This way, by using the tachometer I can compare the calculated speed to the indication on the speedometer. With my 145 with d-type and 80” circumference tires and 4.11 rear end, the speedometer reads 81% lower than the actual speed. It may be coincidence, but the odometer reads about 81% low also based on comparing a 10 mile trip with the odometer on the wife's MPV.

Hope this helps.

--
Tom - '60 544, '68 220S, '70 145S, '86 745T, '06 Mazda MPV