Tire Size Calculator

By Calxy · Calculation methodology

Type the size from your tire sidewall to get its diameter, sidewall height, circumference and revolutions per mile, or compare it with a new size to see the difference and how your speedometer will read.

Printed from Calxy · https://www.calxy.net/other/tire-size-calculator

Diameter difference1.72 %

From the sidewall, e.g. 225/45R17, P215/65R15 or 33x12.50R15.

The speed shown with the original tires fitted and calibrated.

Diameter difference

1.72 %

  • Actual speed at 60 mph indicated61.0 mph
  • Speedometer error-1.0 mph
  • Diameter 225/45R1724.97 in
  • Diameter 235/40R1825.40 in
  • Revolutions per mile 225/45R17807.6
  • Revolutions per mile 235/40R18794.0
225/45R17 vs 235/40R18
Dimension225/45R17 (in)235/40R18 (in)Difference (in)Difference (%)
Section width8.869.250.394.44
Sidewall height3.993.7-0.29-7.16
Rim diameter171815.88
Overall diameter24.9725.40.431.72
Circumference78.4579.81.351.72

Nominal design dimensions from the size code. Real tires differ by brand, rim width, inflation, load and tread wear; check the vehicle placard and a tire professional before changing sizes. A larger tire makes the speedometer read low (you travel faster than shown); a smaller tire makes it read high.

How this was calculated

  1. Sidewall = width × aspect ratio ÷ 100Substitute: 225 mm × 45 ÷ 100Result: 101.25 mm
  2. Diameter = 2 × sidewall + rim × 25.4Substitute: 2 × 101.25 + 17 × 25.4Result: 634.3 mm (24.97244094 in)
  3. Revolutions per mile = 1,609,344 mm ÷ (π × diameter)Substitute: 1,609,344 ÷ (π × 634.3)Result: 807.6148596
  4. Sidewall = width × aspect ratio ÷ 100Substitute: 235 mm × 40 ÷ 100Result: 94 mm
  5. Diameter = 2 × sidewall + rim × 25.4Substitute: 2 × 94 + 18 × 25.4Result: 645.2 mm (25.4015748 in)
  6. Difference = (new diameter ÷ original diameter − 1) × 100Substitute: (645.2 ÷ 634.3 − 1) × 100Result: 1.718429765%
  7. Actual speed = indicated speed × new diameter ÷ original diameterSubstitute: 60 × 645.2 ÷ 634.3Result: 61.03105786 mph

Reading a tire size code

In a metric size such as 225/45R17, 225 is the section width in millimeters, 45 is the aspect ratio (the sidewall height as a percentage of the width) and 17 is the rim diameter in inches. R means radial construction; P and LT prefixes mark passenger and light-truck tires, and load and speed ratings after the size do not change the dimensions.

Sidewall height = width × aspect ratio ÷ 100. Overall diameter = 2 × sidewall + rim diameter × 25.4 mm. Circumference = π × diameter, and revolutions per mile = 1,609,344 mm ÷ circumference. Flotation sizes such as 33x12.50R15 state the diameter (33 in) and width (12.5 in) directly.

Worked example: 225/45R17 to 235/40R18

225/45R17: sidewall 225 × 0.45 = 101.25 mm; diameter 2 × 101.25 + 17 × 25.4 = 634.3 mm (24.97 in); about 807.6 revolutions per mile.

235/40R18: sidewall 94 mm; diameter 645.2 mm (25.40 in); about 794.0 revolutions per mile. The new tire is 1.72% larger, so when the speedometer shows 60 mph you are actually doing 60 × 645.2 ÷ 634.3 = 61.0 mph.

Limits of the calculation

These are nominal design dimensions. A tire's real diameter varies with brand, rim width, inflation, load and tread depth, and a worn tire is several millimeters smaller than a new one.

Many fitters aim to keep a replacement within about 3% of the original diameter, but the right size also depends on load index, clearance and your vehicle's placard, so check with a tire professional.

How to use the Tire Size Calculator

Enter the size from the tire sidewall.

  1. Choose a mode

    Pick one tire size or compare two sizes.

  2. Type the sizes

    Enter codes like 225/45R17, P215/65R15 or 33x12.50R15.

  3. Check the speed

    For a comparison, enter a speedometer reading to see your actual speed with the new tires.

References

Frequently asked questions

Does a bigger tire make my speedometer read high or low?

Low. A larger diameter covers more ground per turn, so you travel faster than the speedometer shows. A smaller tire makes it read high.

What does the 45 in 225/45R17 mean?

It is the aspect ratio: the sidewall is 45% as tall as the tire is wide, so 0.45 × 225 mm = 101.25 mm.

How are revolutions per mile calculated?

Divide one mile (1,609,344 mm) by the tire circumference (π × diameter). A 634.3 mm tire turns about 807.6 times per mile, ignoring the small change from load and tread wear.

Last updated . Results are estimates for informational purposes only.