235/65R17 vs 235/55R18
Diameter -2.93% · Speedo at 60 mph reads 58.2 mph actual · Ground clearance -10.8 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 235/65R17) reads 60 mph, the actual speed with 235/55R18 mounted is:
Speedometer error: -2.93% · Odometer reads low by 2.93%
Full comparison
| Measurement | 235/65R17 | 235/55R18 | Delta |
|---|---|---|---|
| Overall diameter | 29.03" | 28.18" | -2.93% |
| Section width | 235 mm | 235 mm | +0 mm |
| Sidewall height | 152.75 mm | 129.25 mm | -23.50 mm |
| Rim diameter | 17" | 18" | +1" |
| Circumference | 2316 mm | 2248 mm | -68 mm |
| Revs / mile | 695 | 716 | 21 |
| Ground clearance change | -10.8 mm | ||
Questions about this swap
What is the diameter difference between 235/65R17 and 235/55R18?
235/55R18 is 2.93% smaller in overall diameter than 235/65R17 — a difference of 21.6 mm (0.85 inches).
How does 235/55R18 affect the speedometer if the car was calibrated for 235/65R17?
When the speedometer reads 60 mph (calibrated for 235/65R17), the actual speed with 235/55R18 is 58.2 mph — an error of -2.93%.
Is it safe to replace 235/65R17 with 235/55R18?
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.