Example 1
- A: 225/45R17
- B: 225/40R18
634.3 mm / 637.2 mm
How do 225/45R17 and 225/40R18 differ? B is 0.46% taller. At a 100 reading that is 100.46.
Tire Size Calculator
The calculator finds tire diameter from width, aspect ratio and rim, then compares two sizes. 225/45R17 is 634.3 mm. 225/40R18 is 637.2 mm, 0.46% taller.
Pressure in bar and psi is on bar to psi. This is not a tire-set cost.
Result
Enter width, aspect ratio and rim for both tires. The diameters will appear here.
How it works
The calculator finds diameter in millimetres: rim inches times 25.4 plus two sidewalls. A sidewall is width times aspect ratio over 100. 225/45R17 is 17 × 25.4 + 2 × (225 × 0.45) = 634.3 mm.
225/40R18 is 18 × 25.4 + 2 × (225 × 0.40) = 637.2 mm. That is 2.9 mm and 0.46% taller than 225/45R17.
Speed at a 100 reading treats tire A as stock. When B is 0.46% taller, true speed is 100.46 if the speedometer shows 100.
The 225/45R17 stamp is width in mm, aspect ratio in percent and rim in inches. The R means a radial carcass and is not part of the formula.
The calculator does not accept negative or zero sizes. The calculator does not check load index, type approval or arch clearance.
This is not a winter-tire cost and not pressure. Bar and psi belong on the bar-to-psi converter.
Formula
Ø_mm = rim×25.4 + 2×(width×aspect/100); v = 100 × ØB / ØA
Rim is in inches. Width is in millimetres. Aspect is a percent. v is speed at a 100 speedo reading.
How to use
- On tire A enter 225, 45 and 17.
- On tire B enter 225, 40 and 18. The diameters are 634.3 mm and 637.2 mm.
- The change is +0.46%. At a 100 reading the result is 100.46.
- To compare a size with itself, copy A into B. The change stays 0%.
634.3 mm at 225/45R17
A 17-inch rim is 431.8 mm. Two sidewalls of 225 × 0.45 are 202.5 mm. The sum is 634.3 mm.
- 225
- Tread width in millimetres. It enters the sidewall: 225 × aspect/100.
- 45
- Aspect ratio: sidewall = 45% of width. 225 × 0.45 = 101.25 mm on one side.
- R17
- A 17-inch rim, which is 431.8 mm. R does not change the diameter.
Examples
Example 2
- A: 205/55R16
- B: 205/55R16
631.9 mm / 631.9 mm
What happens when both sizes are 205/55R16? The diameters match, and the change is 0%.
Example 3
- A: 195/65R15
- B: 205/55R16
634.5 mm / 631.9 mm
Is 205/55R16 close to 195/65R15? B is 0.41% shorter. At a 100 reading that is 99.59.
Related calculators
Frequently asked questions
What is the diameter of 225/45R17?
634.3 mm: 17 × 25.4 plus two 101.25 mm sidewalls.
What is the diameter of 225/40R18?
637.2 mm. That is 0.46% taller than 225/45R17.
What does 100.46 at a 100 reading mean?
Tire B is taller, so at a 100 indication you travel 100.46. That is a simplified result based on diameters alone.
Is the letter R included in the formula?
No. It marks a radial carcass.
Is this a type approval or a load index?
No. The calculator only finds diameter from three numbers.
Is 205/55R16 the same diameter as 195/65R15?
Almost. 205/55R16 is about 0.41% shorter.
What unit is the rim in?
Inches, as on the sidewall. The calculator multiplies by 25.4.
Where do I convert pressure?
On the bar-to-psi page.
Sources
ISO 4000-1 stamp: width in mm, aspect in %, rim in inches.
Page updated in 2026.