Free tool

Tyre Size Comparison Calculator

Compare two tyre sizes side-by-side — overall diameter, speedometer error, ride height and more. Stay within 3% of the original diameter.

Format: width / profile R rim — e.g. 205/55R16. Results update automatically after the first compare.

Comparison results

Enter two sizes and compare

How to read a tyre size

The width / profile R rim format, using 225/45R17 as the example.

Every tyre carries its size on the sidewall in a standard format. Breaking down 225/45R17:

225

Width

Section width in millimetres, from sidewall to sidewall.

45

Aspect ratio

Sidewall height as a percentage of the width — 225 × 0.45 = 101mm.

R

Construction

Radial — standard for modern tyres. You may rarely see D for diagonal/bias-ply.

17

Rim diameter

The wheel diameter in inches that the tyre fits.

Why tyre size matters

Changing size affects more than appearance — five things to consider.

Speedometer accuracy

Your speedometer is calibrated to your original tyre size. Larger tyres mean you travel further per wheel rotation, so you're actually going faster than displayed.

Fuel economy

Wider tyres have more rolling resistance, which can reduce fuel economy. Larger diameter tyres may also affect gearing and engine efficiency.

Handling & comfort

Lower profile tyres (smaller aspect ratio) offer sharper handling but a firmer ride. Higher profile tyres are more comfortable but may feel less responsive.

Safety systems

ABS, traction control, and stability systems rely on accurate wheel speed data. Significantly different tyre sizes may affect their calibration.

Clearance issues

Larger tyres may rub against wheel arches, suspension components, or bodywork — especially when turning or over bumps.

Plus sizing

Bigger wheels, lower profile, same overall diameter.

Plus sizing means fitting larger wheels with lower profile tyres while maintaining a similar overall diameter. This is popular for improved looks and handling.

Common plus sizing examples

Original Plus 1 Plus 2
195/65R15 205/55R16 215/45R17
205/55R16 225/45R17 235/40R18
225/45R17 235/40R18 245/35R19

Benefits

  • Improved handling and grip
  • Better aesthetics (bigger wheels)
  • Shorter braking distances
  • More responsive steering

Trade-offs

  • Firmer ride (less sidewall)
  • More expensive tyres
  • Higher risk of wheel damage
  • May affect ride quality

Understanding the results

What each measurement means for your car.

Overall diameter

The total height of the tyre when mounted. This is the most important measurement for speedometer accuracy. Keep within 3% of original.

Sidewall height

The height of the rubber between the rim and the tread. Lower = stiffer ride but better handling. Higher = softer ride but less responsive.

Circumference

The distance around the tyre. This directly affects how far you travel per wheel rotation and therefore speedometer accuracy.

Revolutions per km/mile

How many times the wheel rotates to cover 1km or 1 mile. Fewer revolutions with larger tyres means higher actual speed.

The 3% rule: most manufacturers recommend staying within 3% of your original tyre diameter. This keeps speedometer error minimal and ensures proper function of safety systems like ABS and traction control.

Frequently asked questions

Common questions about tyre sizing.

Will changing tyre size affect my speedometer?

Yes. Your speedometer is calibrated to your original tyre size. If you fit larger tyres, your actual speed will be higher than displayed. If you fit smaller tyres, your actual speed will be lower. A 3% change in diameter causes approximately 3% speedometer error.

How much tyre size difference is acceptable?

Most manufacturers recommend staying within 3% of the original tyre diameter. This ensures speedometer accuracy, proper ABS and traction control function, and avoids clearance issues. Larger changes may require ECU recalibration or could affect your insurance.

What does the tyre size 225/45R17 mean?

225 is the tyre width in millimetres. 45 is the aspect ratio (sidewall height as a percentage of width). R means radial construction. 17 is the wheel diameter in inches. So a 225/45R17 has a 225mm width, sidewall height of 101mm (225 x 0.45), and fits a 17-inch wheel.

What is plus sizing for tyres?

Plus sizing means fitting larger wheels with lower profile tyres to maintain the same overall diameter. For example, moving from 205/55R16 to 225/45R17. The benefits include improved handling and aesthetics, while keeping speedometer accuracy and ride height similar.

Do I need to recalibrate my speedometer after changing tyre size?

If you stay within 3% of the original diameter, recalibration isn't usually necessary. For larger changes, some vehicles allow speedometer recalibration through dealer tools. GPS speedometer apps can also help you verify your actual speed.

Will changing tyre size affect my insurance?

Yes, you should inform your insurer of any modifications including tyre size changes. Significantly different sizes may be considered a modification. Failure to disclose could invalidate your policy in the event of a claim.

Can I mix different tyre sizes on my car?

It's generally not recommended to mix different tyre sizes. If you must, tyres of the same size should be on the same axle. Significantly different sizes can affect handling, ABS, and traction control. Always check your vehicle manufacturer's recommendations.

Check the full vehicle history

Sorted your tyre sizes? Check the car's MOT history, mileage trends and any recorded defects with our free vehicle check.

Check a vehicle
Standard metric tyre sizing No registration required Instant, free results