Your speedometer is lying to you. Here’s why

If you’ve ever driven with a standalone navigation system turned on (GPS/phone), you might have noticed that the speed shown on the car’s speedometer reads higher than the speed shown in the navigation system.

Which one is right? Which one do you follow to avoid getting a summoned for speeding?
Well, the truth is, your car’s speedometer is overreporting your speed. And it is doing so intentionally.

Even when conducting the 0-100km/h sprint tests at WapCar. We usually notice a 10% gap between the actual speed (reported by our Racelogic performance box) and the car’s speedometer. And it is always the car’s speedometer that shows the higher reading.

So, what’s the deal with speedometers overreporting the actual speed?

It’s simple, really – Regulation. And we'll get onto that in a while.

Also Read: Should Malaysia’s national speed limit be increased to above 110 km/h?

Is the speedometer or wheel speed sensor design flawed?

Not really, but there's a limit to how accurate things can be designed to be. And the wheel speed sensor is as accurate it gets.

Besides the speedometer, the wheel speed sensor also feeds information to the odometer (held to a higher accuracy standard) and the ABS & ESC control unit.

Also Read: Why is my ABS warning light on? And what is a wheel speed sensor?

The way the speedometer works is by applying simple mathematics to the circumference of the tyre.

Also Read: A "cheap tyre” is going to kill you, here is why good tyres costs more

Over-inflation, under-inflation, or simple wear and tear of the tyre will cause a small change in circumference and this can have a big impact on the speedometer’s readout.

Thus, in order to be effective, the speedometer has to allow for a margin of error.

So, what does regulation have to do with speedometers overreading?

Regulations require that the indicated speed should never read below the true speed of the vehicle. And since margins of error are usually plus and minus, the safe thing to do for manufacturers is to overreport the speed of the car.

The regulation in question is the UNECE Regulation 39 which is usually subscribed by European car makers.

UN ECE Regulation 39 clearly states that “the speed indicated (on the speedometer) shall not be less than the true speed of the vehicle.”

It also outlined that the indicated speed should not be more than 10% higher than the true speed (with additional 4 km/h) as represented by the following equation:

0 ≤  (V₁ – V₂)  ≤ 0.1V₂ +4 km/h

Where,
V₁ = indicated speed
V₂ = true speed

Are there other standards/regulations besides UN ECE Regulation 39?

There are also other standards like SAE J1226 (written in 1983, stabilized in 2011) which is subscribed by most Japanese and American manufacturers.

SAE J1226 allows speed readings to be ±2% of true speed, on a sliding scale from -1% to as much as +4% (depending on operating temperature and voltage).

This means that at maximum, a speedometer can read up to 6% higher than the true speed, and at minimum it can read as low as 3% below the true speed.

Some countries have localised regulations too, like Australia’s ADR 18, but it generally is in accordance with the UNECE Regulation 39.

How do I check how far off my car’s speedometer reading is from the true speed?

Just compare it with the speed shown on a standalone GPS unit. And test the speed on a flat surface (no elevation). Then, simply compare the indicated speed on your speedometer and the GPS device.

GPS-based speedometers have their own inaccuracy issues too, but that's a story for another day.

Conclusion

So, there’s your answer right there. Speedometers overread due to regulation. And difference should not exceed 10% of the true speed +4 km/h. It’s not such a big deal, and can actually be a plus point in helping you avoid that speeding ticket.

Also Read: Here is why strict speed limit enforcement is better than it sounds