This tire size calculator compares two tire sizes, such as 225/45R17 and 235/40R18. It shows overall diameter, sidewall height, width, circumference and revolutions per mile, and how fast you are really going when your speedometer shows a given speed.
How to read a tire size
In 225/45R17: 225 is the width in millimetres, 45 is the aspect ratio (the sidewall is 45% of the width), R means radial construction, and 17 is the rim diameter in inches.
Tire size formulas
- Sidewall height = width × aspect ratio ÷ 100
- Overall diameter = rim diameter × 25.4 + 2 × sidewall height (mm)
- Circumference = diameter × π
- Revolutions per mile = 1,609,344 mm ÷ circumference
Worked example
A 225/45R17 tire is 634.3 mm (24.97 in) tall. A 235/40R18 is 645.2 mm (25.40 in), which is 1.72% larger. When the speedometer shows 60 mph, the car is actually doing about 61.0 mph.
Plus sizing
“Plus sizing” means fitting a larger rim with a lower-profile tire so the overall diameter stays nearly the same. Keeping within about 3% of the original diameter helps the speedometer, ABS and stability control work as designed.
Frequently asked questions
How much can tire diameter change safely?
A common guideline is within 3% of the original diameter. Larger changes can affect speedometer accuracy, braking systems and clearance.
Do bigger tires make my speedometer read low?
Yes. A larger tire travels farther each turn, so you go faster than the speedometer shows.
Can I put a wider tire on my existing rim?
Only if the rim width is within the range approved for that tire size. Check the tire maker’s specifications.
What do load index and speed rating mean?
They are the numbers and letter after the size, such as 94W. Replacement tires should match or exceed the original ratings.
Related calculators
Sources
Last reviewed: October 2026. For information only; results are estimates; check important figures with a professional.