Tyre size comparison
233 side-by-side comparisons between tyre sizes in common use. Each one shows the rolling diameter change, the resulting speedometer error and how much the sidewall and clearance move.
Comparisons cover plus-sizing onto a larger rim and width changes on the same rim, limited to pairs within 5% rolling diameter of each other — beyond that it is not a swap, it is a different tyre.
Calculated result
- Current diameter
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- Proposed diameter
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- Diameter difference
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- At indicated 100
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A close diameter is not a fitment guarantee. Confirm load rating, rim compatibility, clearances and vehicle guidance before ordering.
Formula and worked example
Sidewall height equals tyre width multiplied by aspect ratio and divided by 100. Overall diameter equals rim diameter in millimetres plus two sidewalls. A 225/45R18 tyre is 659.7 mm tall; a 245/40R18 is 653.2 mm, a difference of -6.5 mm or about -0.98%.
| Size | Sidewall | Diameter |
|---|---|---|
| 225/45R18 | 101.3 mm | 659.7 mm |
| 245/40R18 | 98.0 mm | 653.2 mm |
Common questions
How is tyre diameter calculated?
Convert the rim diameter to millimetres and add twice the calculated sidewall height.
Does a larger tyre change the speedometer?
Yes. A larger rolling diameter travels farther per revolution, so estimated actual speed increases relative to the indicated speed.
What diameter difference is acceptable?
There is no universal approval threshold. Use the geometry as one input, then check the vehicle maker, load and speed ratings, wheel width and physical clearance.
From 15-inch wheels
From 16-inch wheels
From 17-inch wheels
From 18-inch wheels
From 19-inch wheels
From 20-inch wheels
How are these fitment figures calculated?
Fitment North calculates declared wheel and tyre geometry from the dimensions shown on the page, using the same tested method as the wheel-offset and tyre-size calculator. Tyre diameter uses rim diameter plus two calculated sidewalls; offset comparisons split width changes across inner and outer edges before applying the offset difference. Results are measurement aids, not vehicle approval. Confirm ratings, clearances and manufacturer guidance before fitting parts.