245/40R18 vs 255/35R18
Diameter -2.68% · Speedo at 60 mph reads 58.4 mph actual · Ground clearance -8.8 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 245/40R18) reads 60 mph, the actual speed with 255/35R18 mounted is:
Speedometer error: -2.68% · Odometer reads low by 2.68%
Full comparison
| Measurement | 245/40R18 | 255/35R18 | Delta |
|---|---|---|---|
| Overall diameter | 25.72" | 25.03" | -2.68% |
| Section width | 245 mm | 255 mm | +10 mm |
| Sidewall height | 98 mm | 89.25 mm | -8.75 mm |
| Rim diameter | 18" | 18" | +0" |
| Circumference | 2052 mm | 1997 mm | -55 mm |
| Revs / mile | 784 | 806 | 22 |
| Ground clearance change | -8.8 mm | ||
Questions about this swap
What is the diameter difference between 245/40R18 and 255/35R18?
255/35R18 is 2.68% smaller in overall diameter than 245/40R18 — a difference of 17.5 mm (0.69 inches).
How does 255/35R18 affect the speedometer if the car was calibrated for 245/40R18?
When the speedometer reads 60 mph (calibrated for 245/40R18), the actual speed with 255/35R18 is 58.4 mph — an error of -2.68%.
Is it safe to replace 245/40R18 with 255/35R18?
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.