How accurate are motorcycle speedometers?

Have you been passing cars wondering if they are travelling slower than you or that there must be an error with your speedo calibration?

If your motorcycle is straight from the dealership, your speedo is accurate, and you can rely on the information. However, if you have modified or have an older motorcycle, your speedo could not give an accurate reading.

Are speedometers accurate on motorcycles?

It would be reasonable to expect motorcycle speedos to be as accurate as car speedos; they work effectively the same way for both analogue and digital readouts.

There is an interesting law in the UK for speedos based on European law. Your speedo can never underreport your speed. It is an offence ( not sure who gets prosecuted). Your speedo can overreport your speed by not more than 10%.

From experience on the motorway, police will allow this variance but not a chance with a speed camera!

So, is your speedo wrong? Well, UK law certainly has a provision built in to expect your speed to be out of whack.

Motorcycles are not unique in that they get modified, but the modifications for making a smoother rider can result in varying speeds. If you change your tyre to something slightly bigger with more grip, your speedo will record a different speed!

Why are motorcycle speedometers wrong?

It’s not just motorcycles; it’s cars and trucks too. Here’s why. Your speed is dependent on the correct inflation of your tyres, if underinflated, you will show less speed on your speed, and overinflated will show more speed.

OK, it’s marginal, but it makes a difference. When your tyre heats on a hot summer day, the air in your tyre will expand further, increasing the diameter of your tyre, hence a higher speed on your speedo.

The other problem is the urge to fit aftermarket products on your motorcycle; bigger tyres and new sprockets with a slightly larger diameter will all contribute to a cumulative increase in speed.

Are speedometers accurate at high speeds?

The same dynamics apply to travelling at speed. If you have altered the specification of your motorcycle by fitting larger tyres for more grip, you will travel marginally faster.

Travelling at high speeds may worsen the problem, but it certainly will not resolve the issue, and if you are concerned about your speed, you are more likely to get a ticket at high speed. Calibrating your speedo will not be acceptable to the police.

Why are speedometers inaccurate?

Speedometers are inherently inaccurate; they are mechanical. Even if you have a digital display, some part of the recorded speed is coming from s sensor picking up the rotation of a mechanical component.

The variable in tyre diameters and tyres inflation pressure will make a difference.

Many digital speedometers get their signal from a sensor close to the ABS sensor, which wears and receives a lot of punishment. 

So it’s easy to see why motorcyclists and other vehicle drivers should be wary of the speed shown on the speedometer.