How the size of your tyres affects the speedometer reading

In Summary

•Many people have altered the speedometer readings unknowingly just because of a simple tyre upgrade.

•Most car manufacturers have tyre recommendations on the pillars between the front and rear doors on both sides of the car.

Tyre diameter.
Tyre diameter.
Image: BOOSTED AUTO

Not many people stick with the tyres that come with the car they buy. Many are attracted to different attributes that push them to change from the original size to bigger tyres. Some of the reasons for upgrading to bigger tyres are genuine while some are just for looks and fashion.

 This makes it a good idea to compare the pros and cons of upgrading to a larger tyre size before concluding whether it is worth it or not.

A Nairobi saying goes: "freeze and shine" to justify following trends despite the weather; this may also seem to be the case when upgrading to fancy looking tyres on a newly acquired ride: as long as they look fancy or they raise your ground clearance, the setbacks that come with them are not considered.

 Apart from being a unit of the steering system and suspension system, tyres are also a form of the transmission system on the car in what is known as final gear reduction in the differential. On the tyre when fitted with a rim, an imaginary straight line running across from end to end through the centre is known as the OVERAL DIAMETER(OD) and it must not be exceeded by 5% of the height when replacing tyres or else the speedometer will give a wrong reading 

It is from this system that the speedometer reading is calibrated for accurate speed and distance covered. In simple terms, the height of the tyre must remain the same to give a specific circumference while rotating.

For example, a larger rim is mostly fitted with a lower profile tyre to try and not exceed an Overall Diameter of a smaller rim while a smaller rim is usually fitted with a larger profile tyre to maintain the Overall Diameter.

In the factory, your car is fitted with smaller tyres, they cover less distance in one full rotation, larger tyres cover more distance in one full rotation (the main reason why when measuring the same distance between 2 points with the same car gives 2 different distance readings when you change tyre size from small to extremely large), technically your vehicle will be travelling more distance than what is being displayed on the speedometer by around extra 20km/h - the main reason why people have been flagged down by cops and told they have exceeded speed limits yet on the speedometer they were sure they did not go beyond the limit. The most likely reason for such people having exceeded their Overall Diameter is very high.

For this reason, if you have to increase the size of your rim say from 15 to 17 then you must use a tyre with very small tyre wall (profile) in order to maintain the permissible limits of the overal diameter. The best way to know whether you have gone above limits is by using an online tyre calculator. You can use https://tyresize.com/calculator/ but there several of them available online,  key in your current tyre size (what your manufacturer recommends vs what you want to upgrade to) should be able to give you the new variance.

Many people have altered the speedometer readings unknowingly just because of a simple tyre upgrade.

Most car manufacturers have tyre recommendations on the pillars between the front and rear doors on both sides of the car.

Find out if you are in the correct bracket. If your answer is no, then use online tyre size and conversion calculator to know how badly off your speedometer is.

The writer is the owner of Boosted Auto automotive repair shop