Tire Size Calculator
This free tire size calculator compares two tire sizes and shows the difference in overall diameter, sidewall height, circumference and speedometer reading — so you can check whether a new size will fit and how it affects your speedo before buying. Enter your current and new sizes below.
Current tire
225/45R17
New tire
245/40R18
| Measurement | Current | New |
|---|---|---|
| Overall diameter | 634.3 mm | 653.2 mm |
| Sidewall height | 101.3 mm | 98.0 mm |
| Circumference | 199.3 cm | 205.2 cm |
Diameter difference
+3.0%
Within the typical ±3% safe range.
Speedometer error
97.1 km/h
True speed when your speedometer reads 100 km/h.
Estimates for guidance only. Always confirm fitment and clearance with a tire professional before changing sizes.
FAQ
How do I read a tire size like 225/45R17?
225 is the tread width in millimeters, 45 is the aspect ratio (sidewall height as a percentage of width), R means radial construction, and 17 is the wheel diameter in inches. From these you can calculate the tire's overall diameter and circumference.
How much tire size difference is acceptable?
As a general rule, keeping the new tire's overall diameter within about 3% of the original avoids significant speedometer error and clearance issues. This calculator shows the exact percentage difference so you can judge it.
Does a bigger tire make my speedometer wrong?
Yes. A larger overall diameter makes your speedometer read slower than your true speed (and a smaller diameter reads faster). The calculator estimates the speedometer error so you know what to expect.
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