Cars are designed around a specific wheel and tyre size, and often a short list of approved alternatives. The speedometer, ABS, traction control, suspension geometry and bodywork clearances all depend on the tyre. Changing size can affect each one.
Keeping the overall diameter
The key principle is to keep the overall diameter of the wheel and tyre the same, or within a very small tolerance. If you fit a larger wheel, such as moving from 16-inch to 17-inch, you need a tyre with a lower profile so the total diameter stays close to the original. Compare the sizes using a tyre size calculator or ask a fitter. A change in diameter alters speedometer readings and can upset the ABS and stability systems. A bigger tyre can make the speedo read slow, which means you travel faster than the display says.
Plus sizing
Plus sizing means fitting a bigger wheel with a lower profile tyre. It can sharpen handling and improve looks, but it makes the ride firmer and puts the wheel closer to potholes. It also costs more to maintain. See tyre bulge in the sidewall for the downside. Always keep the load index at or above the original. See load index explained.
Wider tyres
A wider tyre can give more grip, but only if the wheel's width suits it and there is enough clearance in the arch. Each tyre size has an approved range of rim widths. Too wide a tyre on a narrow rim or the reverse leads to poor handling and uneven wear. Rubbing at full lock or over bumps damages the tyre.
Approved alternatives
Car makers publish approved sizes for each model and variant; some are listed in the handbook, in the manufacturer's data or via the tyre makers. Check there first. Where a size appears on a different version of the same model, it may still not suit yours, since the brakes, suspension and load differ.
Speed rating and load index must still match
Whatever the size, the tyre must meet or exceed the vehicle's load and speed requirements; see speed ratings explained. Some oversize tyres do not exist with the right rating, which can make a particular combination unsuitable.
Legal, MOT and insurance points
Tyres must be suitable for the vehicle and fit without rubbing. A tyre that fouls the bodywork or suspension can fail an MOT. Modifications that change a vehicle's wheel and tyre setup should be declared to your insurer, as a change not disclosed can cause problems with a claim. Check with your insurer.
Winter wheels
A separate set of winter wheels is often a smaller diameter with a higher profile tyre, which suits poor roads. It is a legitimate approach if the size is approved for your car. See winter vs all-season tyres.
Getting advice
Before you change size, ask a technician to check the options for your car. Give the registration and the current size from the sidewall; see how to read tyre sizes. Our mobile tyre fitting technicians can advise on suitable replacements at your location in areas such as London, Manchester, Birmingham and Bristol. Browse the areas list. Call now for a quote on 0800 772 0355 or book now.
Checklist before you decide
- Check the handbook for approved sizes.
- Keep the overall diameter within a small tolerance.
- Match or exceed the load index and speed rating.
- Check rim width suits the tyre.
- Check clearance on full lock and over bumps.
- Tell your insurer about changes if required.
- Make sure the speedometer remains accurate enough.
A worked example of keeping the diameter
Say your car has 205/55 R16 tyres. The sidewall is about 113mm, so the overall diameter is roughly 632mm. Moving to a 17-inch wheel with a 205/50 R17 tyre gives a sidewall of about 103mm and an overall diameter near 632mm, almost identical, so the speedometer stays accurate. Jumping to a 17-inch wheel with the same 55 profile would give a much larger diameter and cause rubbing and speedo error. Always run the sums or ask a fitter before buying wheels.
Temporary and emergency situations
If the exact size is unavailable in an emergency, a fitter may suggest an approved alternative to get you home, but do not treat it as a permanent answer. Never fit two different sizes on the same axle, and on four-wheel-drive cars avoid any mismatch at all, because the transmission can be damaged. If a tyre has to be a stopgap, agree with the technician how long it can be used and when you will replace it with the right one.
Wheel offset and brake clearance
New wheels are not only about diameter. The offset, which sets how far in or out the wheel sits, affects the steering feel and whether the tyre rubs on the arch or suspension. A larger wheel must also clear the brake calipers. A technician cannot judge these from a driveway alone, so use wheel makers' fitment guides for your exact model.
Frequently Asked Questions
Can I fit bigger wheels?
Possibly, if the overall diameter stays about the same and the load and speed ratings are met.
Will a different size affect my speedometer?
Yes, if the overall diameter changes.
Do I need to tell my insurer?
Changes to wheels or tyres can be treated as modifications, so check with your insurer.
Can you advise on sizes?
Yes. Call now for a quote on 0800 772 0355.
Related Help
Need help with your tyres now?
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