Tire size calculator and comparison

How to use it

1. Type the three numbers from the sidewall of your current tire. On 205/55 R16, the width is 205, the aspect ratio is 55 and the rim is 16. If you are unsure what they mean, read how to read a tire size.
2. Enter the size you want to compare in the second box.
3. Read the comparison table, the speedometer table and the equivalent sizes. To find equivalents for a different wheel size (plus-sizing), change the rim in the third box.

What the numbers mean

• Overall diameter decides how far the car travels per wheel turn. It is the number that matters when you change sizes.
• Speedometer: a larger diameter means you are really going faster than the dial shows; a smaller one, slower.
• Odometer: distance is affected in the same proportion, which can matter for mileage records and service intervals.
• Sidewall height changes ride comfort and how well the tire protects the rim.

The formulas

Diameter (mm) = rim (in) × 25.4 + 2 × width × aspect / 100

Difference (%) = (new diameter / current diameter − 1) × 100

Real speed = indicated speed × new diameter / current diameter

Worked example

A 205/55 R16 measures 631.9 mm. A 225/45 R17 measures 634.3 mm, which is 0.38 % larger. When the speedometer reads 100 km/h (62 mph) you are really doing about 100.4 km/h. Over 10,000 km on the odometer you would really have driven 10,038 km.

Limitations

Sources

• ISO 4000-1, Passenger car tyres and rims — Part 1: Tyres (metric series), for the size marking.
• Geometry and speed relationships as in T. D. Gillespie, Fundamentals of Vehicle Dynamics, SAE International, 1992.

Every formula on this page is covered by automated tests with hand-calculated reference values. Found an error? See the about page. To see how this changes your speed in each gear, use the gear ratio calculator.