How to Tell if Your Car’s Speedometer Is Inaccurate
You depend heavily on your car’s speedometer to be accurate so you know how fast you’re actually driving. An inaccurate speedometer could mean that you’re driving slower than everyone else or you could be further over the speed limit than you think you are.
It’s wise to verify the accuracy of your car’s speedometer every so often. If you’re unsure how to do that, here’s an easy method.
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Reasons why your speedometer is inaccurate
Many things can throw off your speed or the way your speedometer reads it. Damaged wiring or a blown fuse could be enough to through the speedometer out of whack. A malfunctioning sensor or engine control unit could be reporting an erroneous speed. A change in wheel or tire size could even throw off the senor and its calculations.
How to calculate your actual speed
Haynes suggests using the following method to calculate your actual speed to compare to what the speedometer says. Find a highway with mile markers on it that you can drive at a consistent speed for a while. Bring a stopwatch and have a friend clock how long it takes you to travel a certain distance—five miles is a good amount. Jot down what the speedometer said your speed was during that time (using cruise control is ideal).
Divide 60 by the travel time in minutes (with seconds as fractions) and multiply by the number of miles traveled. So, five miles traveled in 5 minutes, 30 seconds would be 60 / 5.5 x 5. That will give you your actual speed.
Calibrating an erroneous speedometer
If the rate actual rate is off by a couple mph, that’s not a huge deal, as long as you keep that discrepancy in mind when you’re driving so you don’t inadvertently speed.
If the difference is more than that, you’ll need to calibrate the speedometer. You can do that in most modern electric speedometers by pressing a certain button that can be located by looking in your car’s owner’s manual.
If that doesn’t rectify the difference, bring your car to a certified mechanic who would be familiar with your particular model.
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