Is Your Speedometer Lying to You?
Car speedometers are legally allowed to read high — but never low — so most show 2–10% faster than you’re actually going. Enter your dash speed and a GPS reading to see your true speed and the error in your gauge.
Frequently asked questions
Why does my speedometer read high?
By regulation a speedometer may read high but never low, so manufacturers build in a safety margin — typically 2–10% over your true speed.
How accurate is a GPS speed reading?
GPS speed is usually accurate to within 1 mph under open sky, which is why a GPS head-up display is a reliable way to see your real speed.