225/45R18 vs 235/40R18
Diameter -2.20% · Speedo at 60 mph reads 58.7 mph actual · Ground clearance -7.3 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 225/45R18) reads 60 mph, the actual speed with 235/40R18 mounted is:
Speedometer error: -2.20% · Odometer reads low by 2.20%
Full comparison
| Measurement | 225/45R18 | 235/40R18 | Delta |
|---|---|---|---|
| Overall diameter | 25.97" | 25.40" | -2.20% |
| Section width | 225 mm | 235 mm | +10 mm |
| Sidewall height | 101.25 mm | 94 mm | -7.25 mm |
| Rim diameter | 18" | 18" | +0" |
| Circumference | 2073 mm | 2027 mm | -46 mm |
| Revs / mile | 777 | 794 | 17 |
| Ground clearance change | -7.3 mm | ||
Questions about this swap
What is the diameter difference between 225/45R18 and 235/40R18?
235/40R18 is 2.20% smaller in overall diameter than 225/45R18 — a difference of 14.5 mm (0.57 inches).
How does 235/40R18 affect the speedometer if the car was calibrated for 225/45R18?
When the speedometer reads 60 mph (calibrated for 225/45R18), the actual speed with 235/40R18 is 58.7 mph — an error of -2.20%.
Is it safe to replace 225/45R18 with 235/40R18?
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.