Tire Size Calculator
Compare two tyre sizes such as 225/45 R17 and 235/40 R18. The calculator gives each tyre's overall diameter, sidewall height, circumference and revolutions per mile or km, the percentage difference, and what your speedometer will read against your real speed after the change.
Tire size calculator
How to use the tire size calculator – compare tyre sizes and speedometer error
- Read your tyre sidewall. A size such as 225/45 R17 means width 225 mm, aspect ratio 45% and rim diameter 17 inches.
- Enter tyre A (your current size). Type the width, aspect ratio and rim diameter of the tyres you have now.
- Enter tyre B (the new size). Type the size you want to compare.
- Read the difference. The result shows the change in overall diameter and what the speedometer reads compared with your true speed.
Tyre size formulas
Sidewall height = width × aspect ratio ÷ 100
Overall diameter = rim diameter × 25.4 + 2 × sidewall height (mm)
Circumference = π × diameter
Worked examples
225/45 R17. Sidewall 101.25 mm, overall diameter 634.3 mm (24.97 in).
235/40 R18. Sidewall 94 mm, overall diameter 645.2 mm (25.40 in), which is 1.72% taller than 225/45 R17. With a speedometer reading of 60 mph, the real speed is 61.03 mph.
225/50 R17 versus 225/45 R17. 656.8 mm versus 634.3 mm, 3.55% taller, past the 3% rule of thumb.
265/70 R17 versus 225/65 R17. 802.8 mm versus 724.3 mm, 10.84% taller, which would change gearing, speedometer and clearance a great deal.
Common sizes
| Size | Overall diameter (mm) | Overall diameter (in) |
|---|---|---|
| 205/55 R16 | 631.9 | 24.88 |
| 215/60 R16 | 664.4 | 26.16 |
| 225/45 R17 | 634.3 | 24.97 |
| 225/50 R17 | 656.8 | 25.86 |
| 235/40 R18 | 645.2 | 25.40 |
| 245/40 R18 | 653.2 | 25.72 |
What changes with a new size
A larger overall diameter lowers the engine speed at a given road speed, like a slightly taller gear, and makes the speedometer and odometer read low. A smaller diameter does the reverse. A lower sidewall gives sharper steering response and a firmer ride, and makes rims more exposed to potholes. Always check the load index and speed rating match or exceed the original tyre, that the wheel offset fits, and that there is clearance in the wheel arch at full lock and full suspension travel.
Speedometer accuracy
Even with the original size, speedometers are usually calibrated to read slightly high. A GPS speed reading is a good check. If you change size, use this calculator to see the true speed, and use the speed distance time calculator to recheck journey times. To see how the new diameter affects engine speed, use the gear ratio calculator.
Frequently asked questions
How do I read a tyre size like 225/45 R17?
225 is the tread width in millimetres, 45 is the aspect ratio (sidewall height as a percentage of width), R means radial construction and 17 is the wheel diameter in inches. The load index and speed rating follow, such as 94W.
How do I calculate overall tyre diameter?
Rim diameter in mm plus twice the sidewall height. For 225/45 R17: 17 × 25.4 = 431.8 mm, plus 2 × (225 × 0.45) = 202.5 mm, giving 634.3 mm (24.97 in).
How much difference in diameter is acceptable?
Tyre guides often suggest staying within about 3% of the original overall diameter, because larger changes affect the speedometer, gearing, ABS and traction systems and may cause rubbing. Check the vehicle handbook.
How does a different tyre size change the speedometer?
The speedometer counts wheel revolutions. A taller tyre travels further per revolution, so the speedometer reads lower than your true speed. With a tyre 1.72% taller, a reading of 60 mph is really 61.03 mph.
Does it change my odometer and fuel economy?
Yes. The odometer under-reads distance with taller tyres, by the same percentage. Wider tyres can also raise rolling resistance and fuel use slightly.
What is plus sizing?
Fitting a larger rim with a lower-profile tyre while keeping the overall diameter close to the original, for example moving from 225/45 R17 to 235/40 R18, which is 1.72% taller.
Does tyre width affect the diameter?
Yes, because sidewall height is a percentage of width. A wider tyre with the same aspect ratio has a taller sidewall, so check the diameter whenever you change width.
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Miles per gallon (US and UK), L/100 km and km/L from distance and fuel.
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Trip fuel cost from distance, economy and price, with cost per person.
Sources and references
About this calculator
Written and maintained by Rashid Mehmood, founder of CalcTen. The formulas are unit-tested against hand-worked examples, and the content is reviewed whenever product sizes or guidance change. Last updated 14 November 2026. See our editorial policy.
Results are estimates based on the values you enter. Real fuel economy, range, power and speed vary with driving style, load, weather and vehicle condition. Check your vehicle handbook and local rules before changing tyres or gearing. Read the disclaimer.