275/40R20 vs 285/35R20
Diameter -2.82% · Speedo at 60 mph reads 58.3 mph actual · Ground clearance -10.3 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 275/40R20) reads 60 mph, the actual speed with 285/35R20 mounted is:
Speedometer error: -2.82% · Odometer reads low by 2.82%
Full comparison
| Measurement | 275/40R20 | 285/35R20 | Delta |
|---|---|---|---|
| Overall diameter | 28.66" | 27.85" | -2.82% |
| Section width | 275 mm | 285 mm | +10 mm |
| Sidewall height | 110 mm | 99.75 mm | -10.25 mm |
| Rim diameter | 20" | 20" | +0" |
| Circumference | 2287 mm | 2223 mm | -64 mm |
| Revs / mile | 704 | 724 | 20 |
| Ground clearance change | -10.3 mm | ||
Questions about this swap
What is the diameter difference between 275/40R20 and 285/35R20?
285/35R20 is 2.82% smaller in overall diameter than 275/40R20 — a difference of 20.5 mm (0.81 inches).
How does 285/35R20 affect the speedometer if the car was calibrated for 275/40R20?
When the speedometer reads 60 mph (calibrated for 275/40R20), the actual speed with 285/35R20 is 58.3 mph — an error of -2.82%.
Is it safe to replace 275/40R20 with 285/35R20?
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.