Tyre Size Comparison Calculator

Compare two tyre sizes side-by-side: overall diameter, speedometer error and ride height. A size within about 3% of the original overall diameter keeps the speedometer within about 3%. This tool compares diameter only: see what it does not check.

Format: width / profile R rim, for example 205/55R16. Results update automatically after the first compare.

Comparison results

Enter two sizes and compare.

The format

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

Width
225 mm
Aspect ratio
45
Construction
R (radial)
Rim diameter
17 in

Width

Section width in millimetres, from sidewall to sidewall.

Aspect ratio

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

Construction

R means radial, which is standard for modern tyres. You may rarely see D for diagonal or bias-ply.

Rim diameter

The wheel diameter in inches that the tyre fits.

Five things to consider

1

Speedometer accuracy

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

2

Fuel economy

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

3

Handling and comfort

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

4

Safety systems

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

5

Clearance issues

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

How it works

Plus sizing means fitting larger wheels with lower profile tyres while keeping a similar overall diameter. It is popular for appearance, and it also changes handling and ride.

Common plus sizing examples

Each figure in brackets is the change in overall diameter from the original size. These are examples only: they do not confirm that a size fits or is approved for a particular vehicle.

Original Plus 1 Plus 2
195/65R15 205/55R16 (-0.4%) 215/45R17 (-1.4%)
205/55R16 225/45R17 (+0.4%) 235/40R18 (+2.1%)
225/45R17 235/40R18 (+1.7%) 235/35R19 (+2.0%)

Benefits

  • Larger wheels can improve appearance
  • Can improve handling and grip, depending on the tyre and vehicle
  • Steering can feel more responsive

Trade-offs

  • Firmer ride (less sidewall)
  • More expensive tyres
  • Higher risk of wheel damage from potholes and kerbs
  • Fit and clearance need checking, including rim width and offset

The measurements

Overall diameter

The total height of the tyre when mounted, calculated from the size markings. It sets the speedometer error. A common rule of thumb is to stay within about 3% of the original.

Sidewall height

The height of the rubber between the rim and the tread. A lower sidewall generally gives a firmer ride and sharper steering response. A higher one generally gives a softer ride.

Circumference

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

Revolutions per km or mile

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

How this is calculated

For a size written as width / aspect R rim, the tool uses these formulae:

  • Overall diameter (mm) = rim × 25.4 + 2 × (width × aspect ÷ 100)
  • Circumference = π × overall diameter
  • Revolutions per km = 1,000,000 ÷ circumference in mm. Revolutions per mile = 1,609,344 ÷ circumference in mm.
  • Speed ratio = new overall diameter ÷ original overall diameter. Actual speed = speed shown × speed ratio.

Worked example. 205/55R16 is 16 × 25.4 + 2 × (205 × 55 ÷ 100) = 631.9 mm. 225/45R17 is 17 × 25.4 + 2 × (225 × 45 ÷ 100) = 634.3 mm. That is +0.38% in diameter, so 70 mph on the original speedometer would be about 70.3 mph actual speed.

Larger tyres make the speedometer read lower than your true speed, and smaller tyres make it read higher. For example, a tyre 3% larger in diameter means a speedometer showing 70 mph is about 72.1 mph actual speed.

These are nominal diameters calculated from the size markings, not measured ones. They ignore differences between brands and models, rim width, tyre wear and load.

What this tool does not check

The tool compares overall diameter only. It does not check:

  • Load index or speed rating
  • Rim width and offset, or whether the tyre suits the rim
  • Clearance to the wheel arches, suspension and bodywork
  • ABS, ESC, traction control and other vehicle systems that use wheel speed
  • Whether the size is approved for your vehicle
  • Whether the tyres are legal for road use or would pass an MOT
  • Your insurer's requirements

A result within 3% does not confirm a size is safe or suitable to fit. Check the vehicle handbook or a tyre specialist.

The 3% rule of thumb

A common rule of thumb is to keep the overall diameter within about 3% of the original. A change of that size shifts the speedometer by about 3%. It is a guide, not an approval for a particular vehicle.

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?

A common rule of thumb is to stay within about 3% of the original overall tyre diameter, which keeps the speedometer error to about 3%. It is a guide, not an approval: this tool compares diameter only and does not check load index, speed rating, clearance or vehicle systems such as ABS. Check the vehicle handbook or a tyre specialist. Larger changes may need recalibration on some vehicles.

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 a similar overall diameter. For example, moving from 205/55R16 to 225/45R17, which changes the overall diameter by +0.4%. It is often chosen for appearance, and handling and ride also change, depending on the tyre and vehicle. Because the overall diameter is similar, the speedometer error stays small.

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

A size within 3% of the original diameter changes the speedometer reading by about 3% or less. Whether recalibration is needed depends on the vehicle, so check the handbook or ask a dealer. 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?

Tell your insurer about changes to wheels or tyres, as some insurers treat them as modifications. Not disclosing a modification could affect a claim.

Can I mix different tyre sizes on my car?

It is 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.

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