GPS vs. Your Car's Speedometer: Which One is Actually Accurate?

Your car says 70 MPH. A GPS speedometer says 67. Which one is right?

Usually, the GPS reading is closer to your actual ground speed. That is not because your car is broken. It is because most vehicle speedometers are designed to read a little high.

Modern speedometers calculate speed from wheel rotation and tire circumference. That works well, but it depends on assumptions staying true: original tire size, correct pressure, no drivetrain changes, and no wear that changes effective diameter.

There is also a regulatory reason for overread. In many markets, speedometers are allowed to read high but not low. Start Rescue’s overview summarizes the basic rule, and Geotab explains why real-world discrepancies are common.

Why GPS Is Often Better for Verification

GPS measures speed differently. Instead of estimating from wheel rotation, it calculates movement from satellite data, which is why it is usually more accurate for checking whether your factory speedometer is optimistic. Science ABC and HUDWAY both break down that difference well.

GPS also ignores the usual mechanical variables. Tire changes, tire wear, pressure differences, and some drivetrain modifications can throw off the dash reading, but they do not change how satellite-based speed is calculated.

That said, GPS is not magic. Accuracy depends on signal conditions, update rates are usually slower than a vehicle’s own systems, and it is best used as an independent reference, not as a racing instrument. Urban canyons, weather, and lower refresh rates all matter.

Where Speedometer Error Shows Up Most

Tire size is one of the biggest causes of error. Change diameter enough and your speedometer can be off by several MPH at highway speeds.

That is why GPS verification is so useful for:

• JDM and Euro imports with different regulations or unit conventions
• Older vehicles with mechanical speedometers that drift over time
• Modified cars and trucks running non-OEM wheels, tires, or gearing
• Track day prep, where you want to verify calibration before driving hard

The Privacy Trade-Off

The catch is that most GPS speedometer apps collect location data. You get a better reference, but you often give up privacy in the process.

That is the reason I built eyeSpeed differently. It gives you GPS-based verification within realistic limits, but the calculations stay on-device. No route history leaves your phone, and eyeSpeed is one of very few speedometer apps with a “Data Not Collected” App Store label.

If you want to check your car’s accuracy, compare your dash reading to GPS at steady speeds on safe roads, and make sure your tire size and modifications match what you think they do.

Driving It Home

Your car’s speedometer is often intentionally conservative. GPS is usually the better tool for verification, especially if your vehicle is old, imported, or modified.

Just do not confuse “better verification” with “privacy by default.” Most apps still want your location data. You should not have to hand that over just to find out how fast you are really going.