205/50R17 vs 255/35R18
Swapping from 205/50R17 to 255/35R18 moves you up 1 inch in rim size and changes section width by +50 mm.
Rolling diameter changes by only -0.17%, which is within the variation you would see between a new and a worn tyre. This is a straight swap on rolling diameter alone.
Side by side
| Measurement | 205/50R17 | 255/35R18 | Change |
|---|---|---|---|
| Section width | 205 mm | 255 mm | +50 mm |
| Aspect ratio | 50% | 35% | -15 |
| Rim diameter | 17 in | 18 in | +1 in |
| Sidewall height | 102.5 mm | 89.3 mm | -13.3 mm |
| Rolling diameter | 636.8 mm | 635.7 mm | -1.1 mm (-0.17%) |
| Recommended rim width | 7 in | 9 in | +2.0 in |
Speedometer difference
The proposed tyre is smaller, so it travels less far per turn. Using the original tyre size as the calibration baseline, the calculated road speed is lower than the indicated speed.
| Speedometer shows | You are actually doing | Difference |
|---|---|---|
| 30 km/h | 29.9 km/h | -0.1 km/h |
| 50 km/h | 49.9 km/h | -0.1 km/h |
| 60 km/h | 59.9 km/h | -0.1 km/h |
| 70 km/h | 69.9 km/h | -0.1 km/h |
| 80 km/h | 79.9 km/h | -0.1 km/h |
| 100 km/h | 99.8 km/h | -0.2 km/h |
| 120 km/h | 119.8 km/h | -0.2 km/h |
Clearance
Holding offset constant at ET45 and fitting each tyre to its recommended rim width, the outer face of the wheel moves +25.4 mm and inner clearance changes by -25.4 mm. Sidewall height changes by -13.3 mm, which is what determines whether the tyre catches the arch liner on full lock or compression.
These are geometric figures for the tyre sizes alone. They do not account for your specific vehicle's arch shape, suspension travel or strut clearance. Measure before you buy.
Try your own numbers
The wheel offset and tyre size calculator takes wheel width and offset as well, so you can model a wheel change alongside the tyre change.
Related comparisons
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.