245/45R18 vs 255/40R18
Diameter -2.44% · Speedo at 60 mph reads 58.5 mph actual · Ground clearance -8.3 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 245/45R18) reads 60 mph, the actual speed with 255/40R18 mounted is:
Speedometer error: -2.43% · Odometer reads low by 2.43%
Full comparison
| Measurement | 245/45R18 | 255/40R18 | Delta |
|---|---|---|---|
| Overall diameter | 26.68" | 26.03" | -2.44% |
| Section width | 245 mm | 255 mm | +10 mm |
| Sidewall height | 110.25 mm | 102 mm | -8.25 mm |
| Rim diameter | 18" | 18" | +0" |
| Circumference | 2129 mm | 2077 mm | -52 mm |
| Revs / mile | 756 | 775 | 19 |
| Ground clearance change | -8.3 mm | ||
Questions about this swap
What is the diameter difference between 245/45R18 and 255/40R18?
255/40R18 is 2.44% smaller in overall diameter than 245/45R18 — a difference of 16.5 mm (0.65 inches).
How does 255/40R18 affect the speedometer if the car was calibrated for 245/45R18?
When the speedometer reads 60 mph (calibrated for 245/45R18), the actual speed with 255/40R18 is 58.5 mph — an error of -2.43%.
Is it safe to replace 245/45R18 with 255/40R18?
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.