Example 1
- A: 225/45R17
- B: 225/40R18
634.3 mm / 637.2 mm
How do 225/45R17 and 225/40R18 differ? Tire B is 0.46% taller. When the speedometer shows 100, you are traveling at 100.46.
Tire Size Calculator
The calculator finds the tire diameter from the width, the aspect ratio, and the rim, then compares two sizes. A 225/45R17 tire has a diameter of 634.3 mm. A 225/40R18 tire has a diameter of 637.2 mm, which is 0.46% taller.
Result
Enter the width, the aspect ratio, and the rim for both tires. The diameters appear here.
Pressure in bar and psi is on bar to psi. This is not the cost of a set of tires.
How it works
Tire A225/45R17
Tire B225/40R18
Change+0.46%
The dark ring is the sidewall: 101.25 mm and 90 mm. The light center is the rim. At a speedometer reading of 100 you are traveling at 100.46.
The calculator finds the diameter in millimeters: rim inches times 25.4, plus the two sidewalls. Sidewall height is the width multiplied by the aspect ratio and divided by 100. For 225/45R17 the sum is 17 × 25.4 + 2 × (225 × 0.45) = 634.3 mm.
For 225/40R18 the sum is 18 × 25.4 + 2 × (225 × 0.40) = 637.2 mm. That is 2.9 mm taller than 225/45R17, or 0.46%.
Replacing 225/45R17 with 225/40R18 makes the rim one inch larger and drops the aspect ratio from 45 to 40. The diameter grows by 2.9 mm, which is 0.46%. Revolutions per mile fall from 807.6 to 803.9. The radius grows by 1.45 mm.
The calculator uses tire A as the reference size. When tire B is 0.46% taller and the speedometer shows 100, you are traveling at 100.46.
The code 225/45R17 gives the width in millimeters, the aspect ratio as a percent of that width, and the rim in inches. The letter R means a radial tire and is not part of the formula.
Every measurement has to be greater than zero. The calculator does not check the load index, whether the tire is approved for the car, or whether it rubs the fender.
This is not the price of winter tires, and it is not a pressure calculation. For bar and psi, use the bar-to-psi converter.
Formula
Ø_mm = rim×25.4 + 2×(width×aspect/100); v = 100 × ØB / ØA
The rim is in inches, the width is in millimeters, and the aspect ratio is a percent of the width. The letter v stands for the speed at a speedometer reading of 100. Circumference is pi times the diameter. Revolutions per mile are the 1,609,344 millimeters in a mile divided by the circumference in millimeters. The radius is half the diameter.
How to use
- For tire A, enter 225, 45, and 17.
- For tire B, enter 225, 40, and 18. The diameters are 634.3 mm and 637.2 mm.
- The difference is +0.46%. At a speedometer reading of 100 you are traveling at 100.46.
- To compare a size with itself, enter the same numbers for tire B. The difference is 0%.
634.3 mm at 225/45R17
A 17-inch rim is 431.8 mm. Two sidewalls of 101.25 mm each add up to 202.5 mm. The total is 634.3 mm.
- 225
- Section width in millimeters. Sidewall height is that width multiplied by the aspect ratio and divided by 100.
- 45
- An aspect ratio of 45 means the sidewall is 45 percent of the width. That is 101.25 mm on one side.
- R17
- The rim is 17 inches, which is 431.8 mm. The letter R means a radial tire and does not change the diameter.
Examples
Example 2
- A: 205/55R16
- B: 205/55R16
631.9 mm / 631.9 mm
What does the calculator show when both tires are 205/55R16? The diameters match, and the difference is 0%.
Example 3
- A: 195/65R15
- B: 205/55R16
634.5 mm / 631.9 mm
Is 205/55R16 close to 195/65R15? Tire B is 0.41% shorter. When the speedometer shows 100, you are traveling at 99.59.
Related calculators
Frequently asked questions
What is the diameter of 225/45R17?
The diameter is 634.3 mm. A 17-inch rim is 431.8 mm, and two sidewalls of 101.25 mm each add up to 202.5 mm.
What is the diameter of 225/40R18?
The diameter is 637.2 mm. That is 0.46% taller than 225/45R17.
What does 100.46 mean when the speedometer shows 100?
Tire B is taller, so when the speedometer shows 100, you are traveling at 100.46. This only reflects the difference in diameter.
Is the letter R included in the formula?
No. The letter R means a radial tire and is not part of the formula.
Does this approve the tire or show a load index?
No. The calculator only finds the diameter from the width, the aspect ratio, and the rim.
Is 205/55R16 the same diameter as 195/65R15?
Almost the same. A 205/55R16 is 0.41% smaller in diameter.
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
The ISO 4000-1 size code uses width in millimeters, aspect ratio in percent, and rim diameter in inches.
Page updated September 30, 2026.