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How Accurate is Your Speedometer?

By Dave Lapham

This Tech Tip was borrowed from "Healey Trails," the newsletter of the North Texas Austin-Healey Club. We thank Dave Lapham and Jerry Wall for its use.

Traveling back to Texas from the Southeastern Healey Classic IV last September, I was hoping to avoid any additional speeding tickets despite my grossly inaccurate speedometer.

Luckily we were mostly driving on Interstate highways, so I could time the passing of the mile markers to get a fairly good indication of speed. This is not a new idea, people have done it for years to check speedometer accuracy. The intended purpose of the markers is to assist the highway patrol and other emergency crews in quickly identifying any location on the highway, but they work quite well for checking speed also.

Computing speed is easily done using the formula "speed = distance divided by time."

For cars we normally think in terms of miles per hour, but timing highway markers will probably give us units of miles per second so we must convert. This isn't too tricky, we only need to change seconds to hours. If there are sixty seconds in a minute, and sixty minutes in an hour, there must be 60 X 60seconds in an hour, or 3,600. If one thinks of speed as equaling 1 mile divided by the number of seconds it takes to go that mile, and then multiply that number by 3,600 to change it from miles per second to miles per hour, we have our answer. Confused? Okay, now you know why I'm not a teacher. How about an example?

First an easy one. If we time our travel from one mile marker to the next and determine it takes exactly 60 seconds, we end up with an equation like this: (1 mile/60 seconds) X (3,600 seconds/hour) = 60 miles/ hour. What we've really done is divide 3,600 by 60 (the 60 seconds it took to travel one mile in our example). Let's try another example. If our time to go one mile is 54.5 seconds, we divide that into 3,600 and find that we're traveling 66 miles per hour.

There are two things that make all this easier than it sounds so far:

  1. I use a cheap digital stopwatch to time the mile markers. The lap feature allows me to let the stopwatch run for several miles but get interim results. Also, I can just press the start/stop button without taking my eyes off the road, and once it's stopped, I can wait for a convenient time to read it.

  2. I actually just remember a few key times and don't really do much calculating in my head. To help in that approach, I've put in a table below to help minimize the actual number crunching involved.

If you like, you can just remember a few points from this table and estimate the other points. You could also tape a copy of this to the upper frame of your windshield for easy reference.

By the way, a digital stopwatch should only cost around five or six dollars!

Jerry Wall's Note: When you make your chart, plug in the tachometer readings for several of the most common traveled MPHs and you will find you can depend on your tach more than the speedometer.

How Accurate Is Your Speedometer?

By Norman Nock

Originally published in the Austin-Healey Magazine, April 1991

Here is an easy way to check your speedometer. Road test your car on the highway to check the odometer's accuracy against the speedometer highway markers.

Check the TPM (turns per mile) number on your speedometer. The number is normally in the center of the gauge (i.e. 980, 100, or 1325). The TPM number shows the amount of times that the cable will rotate in one mile. See figure 1.

To check how many TPMs your cable is doing you must first mark 52.80 feet (1/100 of a mile) on a level surface such as a garage floor or a driveway. Remove the speedometer cable from the rear of the speedometer unit. Fit a cardboard counter on the inner flex of your speedometer cable as shown in figure 2. Count the number of turns the counter makes when you push the car the marked distance of 52.80 feet (52 feet, 91/2 inches). Then multiply the number of turns by 100. This number should equal the TPM number marked on your speedometer face plate.

If the TPM number does not equal the turns of the cable, your problem could be:

  • Incorrect speedometer gauge for vehicle

  • Incorrect tire size

  • Incorrect size wheels

  • Incorrect rear axle ratio

  • Fault at cable drive end

  • Drive that is on/or to the main shaft is faulty

Check for a kink in the inner flexible shaft by rolling it on a clean flat surface. Kinks can be seen and felt as shown in figure 3.

Not what you were looking for? Don't forget you can check our back issues using the AHCUSA Magazine Index.

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