Your Speedometer is (Probably) Wrong

Did you know that the speedometer in your car is probably wrong? And, even if you did, did you know that many auto manufacturers mis-calibrate them on purpose?

The United States federal regulations about speedometers are surprisingly vague and hard to find, but it appears that they permit an error up-to five percent of the vehicle’s indicated speed range, plus or minus. European Union (E.U.) regulations prohibit manufacturers from indicating a speed lower than the actual speed, and an up-to five percent error on the plus side. Since most auto manufacturers sell cars in Europe and the United States without major mechanical or electronic changes, they will often intentionally calibrate their speedometers to read high so they can be sure not to offend European regulators.

This is actually the recommended practice, according to the Society of Automotive Engineers (SAE). Their standard for electric speedometer accuracy, J1226, recommends either a calibration within two percent (plus or minus) of the total indicated speed range, or a calibration within four percent with a bias toward reading high. And keep in mind that this is percentage of the total indicated speed range, not percentage of the actual speed. My Subaru Outback has a speedometer with a dial that goes [absurdly] to 150 miles per hour . . . so my speed could read high by up to six miles per hour without running afoul of U.S., E.U., or SAE guidelines.

If you ever compare the speed indicated on your speedometer to the speed indicated on a GPS device, you’ll probably find that the speedometer says you’re going a bit faster than you really are. In my Outback, the discrepancy is about three miles per hour. And the depressing thing is that the car’s on-board computers actually do know the correct speed, they just display it incorrectly on the gauge. If you connect an on-board diagnostics (OBD) scanner to your car, you can display the speed that your car actually thinks it’s going, and it will probably be pretty accurate. There will be some minor variance due to tire inflation and wear (and if you have made aftermarket changes to your wheel or tire size, all bets are off) . . . but my OBD-reported speed is within 0.2 miles per hour of what the GPS reports.

This is part of why I’ve just done a semi-permanent ScanGauge II install in my Outback. In addition to advanced trip statistics, performance monitoring, and economy measurements, I can also display my actual speed . . . something that the car really ought to be able to do on its own.

Scott Bradford has been putting his opinions on his website since 1995—before most people knew what a website was. He has been a professional web developer in the public- and private-sector for over twenty years. He is an independent constitutional conservative who believes in human rights and limited government, and a Catholic Christian whose beliefs are summarized in the Nicene Creed. He holds a bachelor’s degree in Public Administration from George Mason University. He loves Pink Floyd and can play the bass guitar . . . sort-of. He’s a husband, pet lover, amateur radio operator, and classic AMC/Jeep enthusiast.