225/55R18 vs 235/50R18
Diameter -1.77% · Speedo at 60 mph reads 58.9 mph actual · Ground clearance -6.3 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 225/55R18) reads 60 mph, the actual speed with 235/50R18 mounted is:
Speedometer error: -1.77% · Odometer reads low by 1.77%
Full comparison
| Measurement | 225/55R18 | 235/50R18 | Delta |
|---|---|---|---|
| Overall diameter | 27.74" | 27.25" | -1.77% |
| Section width | 225 mm | 235 mm | +10 mm |
| Sidewall height | 123.75 mm | 117.5 mm | -6.25 mm |
| Rim diameter | 18" | 18" | +0" |
| Circumference | 2214 mm | 2175 mm | -39 mm |
| Revs / mile | 727 | 740 | 13 |
| Ground clearance change | -6.3 mm | ||
Questions about this swap
What is the diameter difference between 225/55R18 and 235/50R18?
235/50R18 is 1.77% smaller in overall diameter than 225/55R18 — a difference of 12.5 mm (0.49 inches).
How does 235/50R18 affect the speedometer if the car was calibrated for 225/55R18?
When the speedometer reads 60 mph (calibrated for 225/55R18), the actual speed with 235/50R18 is 58.9 mph — an error of -1.77%.
Is it safe to replace 225/55R18 with 235/50R18?
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.