Should you check your speedo when you fit new tyres?

The wrong-sized tyres could land you with a speeding fine

The Head of Engineering at the Royal Automobile Club of Queensland (RACQ) has recently warned motorists that if they have new tyres fitted, they may need to consider having the speedometer checked. If you have new tyres fitted, you could drive faster than the speed limit without realising it. I know this sounds strange, but let me explain.

Why you may drive faster on larger tyres

If you have new tyres fitted that are larger than recommended by the car manufacturer, it could mean that your speedometer doesn’t show the real speed at which you are driving. You may think you’re travelling at 60 kph, but you could be moving at nearer 70 kph or faster.

This is because larger tyres move relatively faster than smaller tyres. One revolution of the axle will turn your tyres one revolution. A larger tyre may be, say, 115 cm in circumference, compared to a smaller tyre with a circumference of, say, 100 cm. Though your axle is rotating at the same speed, the larger tyre will move you a greater distance – an extra 15 cm per revolution.

When it is manufactured, a vehicle’s speedometer is calibrated to record the speed based upon the manufacturer’s recommended tyre size. In our example, it records one revolution of the axle as 100 cm. If you are driving with larger tyres, your vehicle records the same speed even though you are moving a greater distance.

So, in our example, you may think you are travelling at 60 kph, but because your tyres are 15% larger, you will be travelling at 69 kph.

How to avoid breaking the speed limit because of new tyres

Australian law says that the speedometer should be calibrated on the safe side. This should be around 10% to 15% faster than your actual speed. In other words, the speed that your speedometer tells you that you are travelling at will be faster than your actual speed. You believe you are travelling at, say, 110 kph and your actual speed is likely to be 100 kph.

So, how do you make sure that your speed is not faster than it should be?

When you have new tyres fitted, the technician should refer to the tyre placard found on your door jamb. This tells us what size tyres match the vehicle. Providing the tyres fitted are the right size as per the manufacturer’s recommendations, then you should not get a misreading on your speedometer. With the right tyre size fitted, if your speedometer tells you are driving within the speed limit, then you should be fine.

Don’t rely on your Sat Nav to tell you the correct speed

I love the Sat Nav, but I would never rely on it to tell me how fast I’m travelling. It may give a good indication, but it is not calibrated to your vehicle or its tyres.

How do you know you need your speedo recalibrated?

Providing you always fit the correct size tyres, your speedometer should retain its accuracy. But, like all mechanical equipment, they do go wrong sometimes. There are two tell-tale signs that you may either be driving on the wrong-sized tyres or that you need your speedo checked for accuracy:

  • Your speedometer and Sat Nav show wildly different speeds of 20% or more discrepancy
  • Other vehicles are travelling either faster or slower, despite your speedo showing you are driving at the speed limit

If you notice either of these two things happening, take your vehicle to your nearest tyre fitter immediately. They will be able to check that your tyres are the correct size for your vehicle, and advise on whether speedometer calibration is needed.

Finally, always make sure that you use a reputable tyre service and that the technician changing your tyres checks the tyre details on the tyre placard.

For a tyre service, you can trust in Brisbane, contact Darra Tyres today.

Keeping your family and fleet safe,

Kevin Wood

About the Author

Kevin has been at the forefront of the tyre industry for over 20 years. Kevin's speciality is in industrial and commercial tyres including the management and upkeep of fleets. Kevin has worked with vehicles his whole career from painting, mechanical, suspension and panel beating he has also spent time in the Australia Army as a driver. He has driven all size of vehicles throughout his career so understands the demands placed on drivers.