245/65R17 vs 255/55R18
Diameter -1.68% · Speedo at 60 mph reads 59.0 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 245/65R17) reads 60 mph, the actual speed with 255/55R18 mounted is:
Speedometer error: -1.68% · Odometer reads low by 1.68%
Full comparison
| Measurement | 245/65R17 | 255/55R18 | Delta |
|---|---|---|---|
| Overall diameter | 29.54" | 29.04" | -1.68% |
| Section width | 245 mm | 255 mm | +10 mm |
| Sidewall height | 159.25 mm | 140.25 mm | -19.00 mm |
| Rim diameter | 17" | 18" | +1" |
| Circumference | 2357 mm | 2318 mm | -40 mm |
| Revs / mile | 683 | 694 | 12 |
| Ground clearance change | -6.3 mm | ||
Questions about this swap
What is the diameter difference between 245/65R17 and 255/55R18?
255/55R18 is 1.68% smaller in overall diameter than 245/65R17 — a difference of 12.6 mm (0.50 inches).
How does 255/55R18 affect the speedometer if the car was calibrated for 245/65R17?
When the speedometer reads 60 mph (calibrated for 245/65R17), the actual speed with 255/55R18 is 59.0 mph — an error of -1.68%.
Is it safe to replace 245/65R17 with 255/55R18?
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.