225/55R17 vs 235/45R18
Diameter -1.56% · Speedo at 60 mph reads 59.1 mph actual · Ground clearance -5.3 mm
Generally safe — within the 3% tolerance
The diameter difference is ≤3%, which is the widely-accepted fitment tolerance for speedometer accuracy and ABS/traction-control calibration. Verify clearance and load rating with a tire professional before installing.
Speedometer effect
When the vehicle's speedometer (calibrated to 225/55R17) reads 60 mph, the actual speed with 235/45R18 mounted is:
Speedometer error: -1.57% · Odometer reads low by 1.57%
Full comparison
| Measurement | 225/55R17 | 235/45R18 | Delta |
|---|---|---|---|
| Overall diameter | 26.74" | 26.33" | -1.56% |
| Section width | 225 mm | 235 mm | +10 mm |
| Sidewall height | 123.75 mm | 105.75 mm | -18.00 mm |
| Rim diameter | 17" | 18" | +1" |
| Circumference | 2134 mm | 2101 mm | -33 mm |
| Revs / mile | 754 | 766 | 12 |
| Ground clearance change | -5.3 mm | ||
Questions about this swap
What is the diameter difference between 225/55R17 and 235/45R18?
235/45R18 is 1.56% smaller in overall diameter than 225/55R17 — a difference of 10.6 mm (0.42 inches).
How does 235/45R18 affect the speedometer if the car was calibrated for 225/55R17?
When the speedometer reads 60 mph (calibrated for 225/55R17), the actual speed with 235/45R18 is 59.1 mph — an error of -1.57%.
Is it safe to replace 225/55R17 with 235/45R18?
Generally safe — within the 3% tolerance. The diameter difference is ≤3%, which is the widely-accepted fitment tolerance for speedometer accuracy and ABS/traction-control calibration. Verify clearance and load rating with a tire professional before installing.
All calculations use standard ETRTO formulas. Confirm fitment with a qualified tire professional and your vehicle's door-placard before purchasing.