A car tyre has a measurement that will look similar to this:
205/60R16
The 205 is the width in millimetres, the R16 means that the hub’s diameter is 16 inches and the 60 refers to the sidewall height as a percentage in relation to the width. In this case, the sidewall (the height of the tyre is 60% of 205mm.
So, there are three measurements which could cause a tyre to be a different size.
Let’s deal with the width first: many sports cars come with different width tyres on the rear than they do on the front. This is because if they are rear-wheel drive, this gives more traction, but such wide tyres at the front could cause ‘tramlining’ where the car wants to follow grooves in the road. It also doesn’t necessarily provide any better turning ability, but does create more rolling resistance (more tyre in contact with the ground) and more weight (heavier tyre). However, you would not put a narrower tyre on one side of the car.
Some vehicles come with wheels of different diameters front and rear – tractors are an obvious example where the rear tyres are massively bigger than the front tyres. Some box vans have a standard-sized front wheel and a dual smaller wheel at the rear. In this case, the manufacturer will have set up the suspension geometry to account for this. Again, you wouldn’t use a different diameter tyre on one side.
You may need to use a different sized tyre in an emergency: your space saver spare wheel. In this case, you will need to keep your speed under the recommended maximum for the tyre (usually 80km/h) and minimise the distance you drive.
What happens if you have a different sized wheel on one side of your car?
Your differential, which allows the wheels to turn at different speeds so that you can more easily go around corners, will be working overtime and damage could occur.
The suspension of your car will be working harder on one side. It may cause the vehicle to want to turn in one direction.
A wheel with a smaller diameter will measure a greater speed and distance travelled in relation to your actual speed, whereas a wheel with a larger diameter is at risk of rubbing on the inner guard.
If the size difference is on the drive wheels, when you accelerate, it will pull towards the side of the smaller wheel.
If your vehicle has electronic stability control or traction control, it is likely to cut in more.
If you have a four-wheel drive vehicle, it’s usual that all the diameters will be the same otherwise the centre differential will wear very quickly.
Should you change two tyres at a time or is just one OK?
Your tyres should wear out roughly evenly – if they don’t you’re either racing in circles, you have a tyre pressure imbalance and one is too low, or something else mechanical is wrong and should be checked (e.g. something rubbing on the tyre).
However, it’s possible to get a puncture at any time – even (annoyingly) when the equivalent tyre on the other side has plenty of wear. Should you replace both of them?
The worn tyre will be a slightly smaller diameter because some of the tread is worn off, so ideally, yes. However, if you choose not to, a tyre shop can balance the tyres as best they can.
Can you fit wider or narrower tyres than the factory installation?
This depends on the range of sizes supported by the vehicle model and the rim. For example, many rims and vehicles will support a tyre that is 10mm larger than the standard specification, e.g. going from a 205/50R17 to a 215/50R17 – even this little bit of extra width can make a positive difference to the handling. However, the rim would not be wide enough to accept a 255/50R17. You will need to check with your local tyre shop.
Are smaller tyres better for acceleration?
Smaller tyres have less inertia and therefore are easier to turn. This will lead to an increase in accelation. However, as the tyres are smaller, the engine will need more revs to maintain its speed over a tyre that’s larger, therefore it will use more fuel.
Darren is an expert on driving and transport, and is a member of the Institute of Advanced Motorists
