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Speedometer Error Calculator
Hold a steady speed, ask a passenger to note both readings at the same moment, then enter them here.
How to Test Your Speedometer With GPS
- 1. Pick the right road
Avoid tunnels, tall buildings, overpasses and sharp bends. GPS is least accurate during acceleration and under obstructed sky.
- 2. Hold a steady speed
GPS updates about once a second. Compare only when the speed has been constant for several seconds.
- 3. Test several speeds
Speedometer error is usually a percentage, so it grows with speed. Three test speeds show the pattern.
- 4. Let a passenger read
Never look at your phone while driving. A passenger can read both values, or you can run the test from a dash mount and check the trip’s figures after stopping.
How Accurate Should a Speedometer Be?
Because under-reading is not allowed, manufacturers build in a small margin, and most new cars read a few percent high. In the United States there is no federal accuracy limit for passenger-car speedometers; manufacturers follow industry practice. Read more in why car speedometers read high.
| True speed | Allowed dashboard range (UN R39) |
|---|---|
| 30 km/h | 30 – 37 km/h |
| 50 km/h | 50 – 59 km/h |
| 80 km/h | 80 – 92 km/h |
| 100 km/h | 100 – 114 km/h |
| 120 km/h | 120 – 136 km/h |
What Makes a Speedometer Inaccurate?
- Tire size: larger-diameter tires than stock make the speedometer read low; smaller ones make it read high. This is the most common cause after modifications.
- Tread wear and pressure: worn or under-inflated tires have a slightly smaller rolling radius, adding about 1–3%.
- Gear or differential changes: a different final-drive ratio changes the reading unless the car is recalibrated.
- Faulty sensors: a failing vehicle-speed or wheel-speed sensor causes erratic or zero readings.
Comparing GPS and dashboard? See GPS speed vs. speedometer: which is more accurate.
Check Any Speedometer: Car, Motorcycle, Bike or Boat
- Car speed test — compare your dashboard at highway speeds
- Motorcycle speedometer — motorcycles often over-read more than cars
- Bike speedometer — check a magnet-based bike computer’s wheel setting
- Boat speedometer — compare a paddle-wheel log with GPS speed over ground
Frequently Asked Questions
How do I check if my speedometer is accurate?
Compare it with GPS at a steady speed on an open road, then enter both values in the calculator on this page.
Is it normal for a speedometer to be off by 5 km/h?
At highway speeds, reading 3–6 km/h (2–4 mph) above true speed is common and legal. Reading below true speed is not normal.
Can a speedometer be recalibrated?
Yes. Modern cars are usually recalibrated in software with a diagnostic tool after a tire-size change; older cars may need a different drive gear or a calibration module.
Is a phone GPS accurate enough to test a speedometer?
At a constant speed in open sky, phone GPS is usually within about 1–2 km/h, which is enough to spot a meaningful speedometer error.