GPS Speed vs Car Speedometer: Which Is More Accurate?
By Mokshith Naik
Developer & Creator of Online Speedometer
Testing Note by Mokshith: I've tested this GPS speedometer against three different car dashboards, and the results surprised me. On a 60 km/h road, my GPS reading was consistently 2–3 km/h lower than the car's display — which is actually normal, because car speedometers are legally required to never underreport speed.
How a Car Speedometer Measures Speed
A traditional vehicle speedometer doesn't measure ground displacement directly. Instead, sensors count how many times the drive shaft or wheels rotate per minute. The car's computer multiplies wheel rotations by tire circumference to calculate speed.
As tires wear down or change pressure, their circumference shrinks, causing mechanical speedometers to over-read.
How GPS Measures Ground Speed
GPS calculates true ground speed by measuring physical distance traveled over satellite time intervals (Speed = Distance / Time). It is completely unaffected by tire wear or mechanical wheel size changes.
About the Author
Mokshith Naik — Creator of Online Speedometer
Mokshith is the developer behind Online Speedometer. He builds privacy-focused web tools and personally tests GPS speed metrics across cycling, driving, and running routes.
✉️ Contact Author: mokshithnaik67@gmail.com