We all know that bigger wheels make cars look better, but is it always a good idea to switch to larger tyres? What about the speedometer and its readings? In this article you will learn about how changing tyre sizes affects speedometer readings, and whether the speedometer discrepancy is significant enough to worry about.

A speedometer displays the car's current speed.The information on the dashboard about the car's current speed is almost never the same as the result calculated from the number of wheel rotations.

How does a car speedometer work?

Before we examine the actual impact of new tyres on speedometer readings, it is worth taking a closer look at how a car's speedometer works. The speedometer is a measuring device that is sometimes directly attached to the car's drive shaft via a special cable. In modern cars, speedometers utilise an advanced sensor that measures wheel revolutions. The more revolutions there are, the higher the reading on the dashboard display. These readings are analysed by a microprocessor and converted into pulses.

In practice, a car's speedometer works in a very simple manner. Sensors on the car's wheels send data to a computer, which converts the data into a numerical value that can be understood by the user.

Is there a link between wheel size and the speedometer reading?

Many drivers consider replacing their wheels with larger ones simply because they want to make their car look more attractive and to give it a more serious character. However, in most cases they do not consider the consequences of this type of tuning. It is worth knowing that larger tyres have a real impact on the driving characteristics of your car.

Often, the speedometer is not calibrated to the specific tyre, which can lead to speedometer discrepancies. In some cases, this can impact the safety of the driver and other road users, making it a common problem with wheel size and speedometer discrepancies.

A close-up of a car wheel on the road.If the tyres are replaced with significantly larger or smaller ones, the speedometer needs to be re-calibrated.

How does increasing the size of your tyres affect the car’s speed? The principle is simple – the speed may also increase. For this reason, new tyres may have up to 3% more circumference than the old ones. Otherwise, the speedometer needs to be re-calibrated. Anything over this limit will result in an erroneous speed reading.

How inaccurate are speedometers?

Car manufacturers intentionally overstate speedometer readings. By how much are speedometer readings overstated? Usually not more than a dozen or so kilometres. This is done for two reasons:

  • Safety – if the speedometer reads 100 km/h, the car is actually travelling slightly slower. In the event of an accident, this minimises its consequences. The European Union has even introduced official recommendations for car manufacturers to slightly overstate speedometer readings. This is because of the psychological aspect of driving, which has a direct impact on road safety.
  • Road checks – a small error in speed measurement can save you from a ticket. Many drivers like to check the performance of their car by stepping on the accelerator and watching the dashboard reading soar. If the resulting measurement slightly distorts reality, this may actually lead to slower driving while still satisfying the motorist’s ”need for speed”.
Yellow speed cameras on roads.One of the reasons for slightly distorting speedometer readings is to help drivers avoid fines after being caught by a speed camera.

Speedometer display error – how to check it?

Using a GPS navigation system is the easiest way to check whether your car's speedometer is displaying the correct speed. In addition to pinpointing your location, most GPS devices can accurately measure your speed as well. However, keep in mind that the accuracy of the readings decreases at lower speeds. They are reliable only when driving at speeds exceeding 40 km/h.

Speedometer discrepancy caused by wheel size – calculator

Many drivers find it difficult to choose new tyres. How do you know whether a particular tyre will fit a specific car model and not lead to distorted readings by the car's speedometer? To find out if your new tyres are compatible, use a specialised tool such as the Oponeo replacement tyre calculator. It will ensure that your tyre selection process is a breeze, and you won’t need to worry about speedometer errors.