WheelSignal

WheelSignal

Results printed from https://wheelsignal.com/speedometer-error-calculator. Nominal estimates; see assumptions below.

Speedometer error calculator

Compare what your speedometer shows with a reference such as GPS to find the percentage error, then read off true speed at any indicated speed.

Speedometer and odometer check

Speed check
Odometer check (optional)
Over a measured route
Same unit as the odometer
Optional reading to convert

Unit choices are saved only if you tick the box, in this browser’s local storage. Measurements are never saved or sent anywhere. A shared link carries the inputs after the # in the address, which browsers don’t send to the server.

Results

Enter an indicated speed and the true (GPS) speed, or press Try an example, then Calculate.

How to use the speedometer error calculator

  1. On a straight, level road, hold a steady speed and note what the speedometer shows and what a GPS app or navigation unit reports. A passenger should take the reading.
  2. Enter both numbers and choose mph or km/h.
  3. Optionally, reset the trip odometer, drive a route of known length (for example between motorway distance markers), and enter both distances.
  4. Press Calculate for the error, correction factor and a table of true speeds.

Worked example

The speedometer shows 70 mph while GPS reports 67 mph. The speedometer over-reads by 3 mph, which is 3 ÷ 67 = 4.48% of the true speed. The correction factor is 67 ÷ 70 = 0.9571, so an indicated 50 mph is about 47.9 mph.

Indicated speed compared with true speedUpper scale shows indicated mph; lower scale shows the true speed at the same needle position. True = indicated × 0.9571.IndicatedTrue0109.62019.13028.74038.35047.96057.47067.08076.6Assumes the error is proportional across the range (mph)

Why speedometers usually over-read

UK vehicle rules, which adopt UN ECE Regulation 39, say a speedometer must never show less than the true speed and, at approval, may show at most 10% of the true speed plus 4 km/h more (Hansard, 2001). So at a true 100 km/h, a reading anywhere from 100 to 114 km/h is within the approval tolerance. Makers aim inside that band, which is why a small over-read is normal.

Tyres matter too. A worn tyre has a slightly smaller rolling circumference than a new one, and a different size changes it more. If you’ve changed tyre size, the tire size calculator estimates the effect from the size markings.

Formulas used

error (%) = (indicated − true) ÷ true × 100 correction factor = true ÷ indicated true speed = indicated × correction factor odometer factor = actual distance ÷ odometer distance

The calculator assumes the error is proportional to speed, which is how a wrong tyre circumference behaves. Some speedometers add a fixed offset as well, so check at more than one speed if you need precision.

Frequently asked questions

Is GPS speed more accurate than my speedometer?

At a steady speed on a straight, level road with a good satellite signal, a GPS receiver’s speed is usually a better reference than a car speedometer. It’s less reliable when accelerating, in tunnels, among tall buildings or on twisty roads. Take several readings at a steady speed and use the average.

Why do most speedometers read high?

Vehicle approval rules in the UK and many other countries (UN ECE Regulation 39) don’t allow a speedometer to show less than the true speed, and limit how far over it may read. Manufacturers therefore calibrate toward over-reading, which also allows for tyre wear and size tolerances.

Is the odometer wrong by the same amount?

Not necessarily. Many modern cars deliberately offset the speedometer display but keep the odometer close to true. Check the odometer separately, ideally against a known distance, using the odometer section of this calculator.

Does a tyre change affect the reading?

Yes. A larger overall diameter covers more ground per revolution, so the true speed rises relative to what’s shown. Use the tire size calculator to estimate the effect of a size change before fitting, then confirm with a measurement here.

Can I use this to recalibrate my speedometer?

The correction factor is the ratio a recalibration tool or programmer typically asks for, but how to apply it depends on the vehicle. Recalibration should follow the vehicle maker’s or equipment maker’s instructions.

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