235/45R17 vs 245/40R17
Diameter -2.41% · Speedo at 60 mph reads 58.5 mph actual · Ground clearance -7.8 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 235/45R17) reads 60 mph, the actual speed with 245/40R17 mounted is:
Speedometer error: -2.42% · Odometer reads low by 2.42%
Full comparison
| Measurement | 235/45R17 | 245/40R17 | Delta |
|---|---|---|---|
| Overall diameter | 25.33" | 24.72" | -2.41% |
| Section width | 235 mm | 245 mm | +10 mm |
| Sidewall height | 105.75 mm | 98 mm | -7.75 mm |
| Rim diameter | 17" | 17" | +0" |
| Circumference | 2021 mm | 1972 mm | -49 mm |
| Revs / mile | 796 | 816 | 20 |
| Ground clearance change | -7.8 mm | ||
Questions about this swap
What is the diameter difference between 235/45R17 and 245/40R17?
245/40R17 is 2.41% smaller in overall diameter than 235/45R17 — a difference of 15.5 mm (0.61 inches).
How does 245/40R17 affect the speedometer if the car was calibrated for 235/45R17?
When the speedometer reads 60 mph (calibrated for 235/45R17), the actual speed with 245/40R17 is 58.5 mph — an error of -2.42%.
Is it safe to replace 235/45R17 with 245/40R17?
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.