How to verify the accuracy of a speedometer

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Car speedometers are not always accurate. Some are as much as 10% off or more. This has several important implications. If your speedometer is reading high, you're probably going slower than you need to, and aggravating drivers behind you. In addition, your odometer reading is probably advancing too quickly, running you out of warranty faster. If your speedometer is reading low, you may well be subject to repeated speeding tickets, costing you money and time. The following will show you how to quickly and easily verify if your speedometer is accurate, and if not, how far off it is and in which direction.

Skill level:
Moderate

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Things you need

  • A watch displaying seconds.
  • A stretch of highway without traffic congestion and with mile markers.
  • A friend to drive the car while you measure time.
  • A pen.
  • A notebook.
  • A calculator.

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Instructions

  1. 1

    Have your friend drive at the speed limit based on your car's speedometer, maintaining constant speed as much as possible.

  2. 2

    Check your watch as soon as you pass a mile marker. Write down the number on the mile marker and the time you passed it at, including seconds.

  3. 3

    Repeat step 2 for the next 5 miles, or even 10 miles if you can. The longer the distance, the more accurate your measurement will be.

  4. 4

    Subtract the number of the first mile marker from that of the last mile marker. If the number is negative, drop the minus sign and make it positive.

  5. 5

    Subtract the time you passed the first mile marker from the time you passed the last one. Translate the difference into seconds.

  6. 6

    Divide the number of miles from step 4 by the number of seconds from step 5. This is your measured average speed in miles per second and should be about 0.0153 if you're going at 55mph.

  7. 7

    Divide your measured average speed by the speed your speedometer was showing in mph throughout the measurement. Multiply the result by 3600. If the final number is 1.0 congratulations, your speedometer is accurate. If the number is higher than 1.0, your speedometer is reading low and you're going faster than you think. If the number is lower than 1.0, your speedometer is reading high, and you're going slower than you think.

  8. 8

    Subtract 1.0 from the number you arrived at in step 7, and multiply the result by 100. This is the percentage error of your speedometer. If the number is +9%, you're travelling 9% faster than you think. If it's -5%, you're travelling 5% slower than you think.

Tips and warnings

  • If your speedometer is off by more than a few per cent, you may wish to ask your dealer to replace it. If the car is still under warranty you can prove to the dealer there is a problem and have them replace it free of charge.
  • Do not carry out the above procedure while driving yourself. You may become distracted and cause an accident.
  • When subtracting times in step 5, remember there are 60 seconds in a minute, not 10 or 100. The safest thing is to translate the time into seconds only, by multiplying the hours by 3600, and the minutes by 60, and then summing those two with the seconds. If all hour readings are the same you can simply drop the hours to make things simpler.

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