Do Speedometers Underreport Your Speed? Myth or Reality

There’s a common belief that a car’s speedometer “lies” to drivers by showing a lower speed than they’re actually traveling. So is that really how it works, or is it just a myth—or maybe even a sign something’s wrong with the car?

In reality, every speedometer is a little off—but not in the way many people think. It doesn’t underreport speed; instead, it slightly distorts the reading due to how the system calculates it. Most vehicles measure speed based on the rotation of the transmission’s output shaft. Because of that, there’s typically a small margin of error—usually around 0.3% to 0.5% from the vehicle’s actual speed.

There are other factors that can affect accuracy, too. Modifications like installing non-stock gears or different-sized wheels and tires can throw off readings. Even everyday issues—like low tire pressure—can influence how precise the speedometer is.

Here’s the key point: engineers and regulators have already accounted for all of this. As a result, speedometers are designed to err on the safe side. Instead of showing a lower speed, they’re calibrated to read slightly higher than your true speed.

That means a properly working speedometer doesn’t understate your speed—it actually overstates it. This approach helps improve road safety and reduces the risk of drivers unknowingly exceeding speed limits and getting ticketed.