How each one measures
A car speedometer infers speed from wheel or transmission rotation, so tire size, wear, and pressure shift it. A GPS speedometer infers speed from satellite positioning, independent of the tires, but dependent on signal quality.
Why the dashboard often reads higher
Vehicle regulations in many markets allow speedometers to over-read but not under-read, so manufacturers typically calibrate them a little high. Worn or non-standard tires change the reading further.
Why GPS can be off
Weak signal, tunnels, urban canyons, and sudden acceleration can make GPS lag or wander for a moment. ClearSpeed shows Weak GPS rather than a guessed number when the fix is poor.
Compare them fairly
- Use a straight road with open sky and hold a steady speed.
- Wait for Good GPS before comparing.
- Make sure both show the same unit (MPH vs. KM/H is the most common "error").
- Have a passenger read the numbers, or compare while parked from a recorded max speed — never while driving.
Which to follow
Drive by your vehicle's speedometer and posted signs. A GPS app is a useful second reference, not a legal one.
Driving safely
Set units, mode, and brightness while parked. Mount the phone securely where it does not block your view and where local law allows, then leave it alone while the vehicle is moving. ClearSpeed is an informational display; your vehicle's speedometer and posted signs remain the reference.
Try it on your next trip
ClearSpeed GPS is $3.99 once in the U.S. App Store, with no subscription, in-app purchases, or ads. View ClearSpeed GPS on the App Store.