215/45R18 vs 225/40R18
Diameter -2.08% · Speedo at 60 mph reads 58.8 mph actual · Ground clearance -6.8 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 215/45R18) reads 60 mph, the actual speed with 225/40R18 mounted is:
Speedometer error: -2.07% · Odometer reads low by 2.07%
Full comparison
| Measurement | 215/45R18 | 225/40R18 | Delta |
|---|---|---|---|
| Overall diameter | 25.62" | 25.09" | -2.08% |
| Section width | 215 mm | 225 mm | +10 mm |
| Sidewall height | 96.75 mm | 90 mm | -6.75 mm |
| Rim diameter | 18" | 18" | +0" |
| Circumference | 2044 mm | 2002 mm | -42 mm |
| Revs / mile | 787 | 804 | 17 |
| Ground clearance change | -6.8 mm | ||
Questions about this swap
What is the diameter difference between 215/45R18 and 225/40R18?
225/40R18 is 2.08% smaller in overall diameter than 215/45R18 — a difference of 13.5 mm (0.53 inches).
How does 225/40R18 affect the speedometer if the car was calibrated for 215/45R18?
When the speedometer reads 60 mph (calibrated for 215/45R18), the actual speed with 225/40R18 is 58.8 mph — an error of -2.07%.
Is it safe to replace 215/45R18 with 225/40R18?
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.