Hi everyone,
I wqws wondering how accurate my speedometer was reading so I was contemplating blasting past a police radar trap and see what speed he was going to ticket me. ...but I figured it may be better to measure my odometer reading against road side mile marker signs and see how that compared instead.
With my current snow tires my odometer reads spot on to the mile marker signs. I have tried this on different roads and the odometer always clicks off a 1/10th of a mile exactly at each marker. If I do several markers or miles it is always spot on.
Does this then translate to an accurate speedometer reading? ( I know it should be close enough... but is it also spot on then?)
Tahnks,
CU

The theory given is great, but just check against the mile markers with a watch. 1 mile = 60 seconds @ 60 mph (you knew that.) Even easier, GPS units are everywhere and make good, accurate speedometers that allow you to check different speeds easily (my experience is that percentage errors often vary between speed ranges, good to know in a school zone too.)
--
67 144, 86 740T, 91 945SE, BMW R69S, R60/2

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

I second the Walrus' comment, but would add that you can, of course, check the speedometer with a stopwatch against a measured mile, or by having someone with a known accurate speedometer follow you on a quiet road and flashing lights at pre-agreed speeds. I know that you can adjust the ribbon type speedometers by changing the tension on the spring at the left of the drum - I don't know about the dial type, if that's what you have - but rather than going through that exercise, I would note the deviations and make mental adjustments as I drive. That's a lot easier.
Bob S.

Thank you everybody.
If I have a passenger I will time my milemarkers or I will follow someone w/ an accurate speedometer. I really just want to know roughly how close Imy car is to accurate and keep a mental note of it.
I was just curious because the odometer was so exact to the mile markers.
Again thanks for all of the suggestions.
CU

Not necessarily but usually if the odo is off the speedometer WILL be off.
There are things that can affect the speedo (gum, wear, etc) that will have
no effect on the odometer.
--
George Downs Bartlesville, Heart of the USA!