No matter what make and model you drive, your speedometer is definitely off. Case in point, at highway speeds it will almost always show a faster rate than you’re actually traveling. And if your car's European, the problem's even worse. Here are the reasons why:
Tiny changes in a tire’s circumference can be caused by over- or under-inflation, or by the wearing-down of tread over time. This can have a big impact on the speedometer’s readout, which means in order to be effective, it has to allow for a margin of error.
The four percent rule
For ordinary passenger vehicles, there’s no law in the U.S. that regulates speedometers, but U.S. manufacturers (and most Japanese as well) subscribe to a standard called SAE J1226. Your speedometer reading must be within a range of plus or minus four percent off, but that’s four percent over the entire range of the speedometer, and that range can then be shifted in what’s called a bias.
Look at the above chart based on a Scion’s technical guide. At low speeds you’re potentially going faster than the speedometer says, but at 100 mph you’re going at least three miles per hour slower.
The European formula
Now it gets really crazy. The European Union requires adherence to UN ECE Regulation 39. It’s a lot of math, but the simple version is that no speedometer can read slower than the actual speed. Ever. On the high side, it’s allowed to read up to 10% above the actual speed plus four or six kilometers per hour, depending on the type of vehicle.
Compare this chart to the first one. The most a speedometer can be off at that same 100 mph (160 kph) is nearly 13 mph. This is why many BMW and Porsche drivers notice their speedometer is off.
So, which one is more accurate?
That depends a lot on how fast you’re going. If you go by the formulas, SAE-calibrated speedometers will definitely be off as you cross into triple digit speeds, and European ones may still be on target. In practice though, for people that don’t constantly stare at a speedometer while driving at high rates of speed, the SAE speed is more likely to read true.
What about my odometer?
That’s held to a tighter standard and is for a different discussion. If you really want an inside look at how manufacturers regard tolerances, there’s some great technical data in a lawsuit over odometer accuracy that Nissan won a few years ago.
How to test your speedometer
There are two ways. You could use a GPS device or get an app for your phone that will measure distance over time. These can be extremely accurate, but not all of them take elevation changes into account.
Or—and this is how the police calibrate their speedometers—use a wristwatch or a stopwatch. If you have a chronograph watch with a tachymeter (the numbers on the rim that go to over 500), you just cruise down the highway at a steady speed, start your stopwatch right as you pass a mile marker, then stop it again when you pass another. Your second hand will be pointing at your exact speed.