275/55R20 vs 285/55R20
Diameter +1.36% · Speedo at 60 mph reads 60.8 mph actual · Ground clearance +5.5 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 275/55R20) reads 60 mph, the actual speed with 285/55R20 mounted is:
Speedometer error: +1.35% · Odometer reads high by 1.35%
Full comparison
| Measurement | 275/55R20 | 285/55R20 | Delta |
|---|---|---|---|
| Overall diameter | 31.91" | 32.34" | +1.36% |
| Section width | 275 mm | 285 mm | +10 mm |
| Sidewall height | 151.25 mm | 156.75 mm | +5.50 mm |
| Rim diameter | 20" | 20" | +0" |
| Circumference | 2546 mm | 2581 mm | +35 mm |
| Revs / mile | 632 | 624 | -8 |
| Ground clearance change | +5.5 mm | ||
Questions about this swap
What is the diameter difference between 275/55R20 and 285/55R20?
285/55R20 is 1.36% larger in overall diameter than 275/55R20 — a difference of 11.0 mm (0.43 inches).
How does 285/55R20 affect the speedometer if the car was calibrated for 275/55R20?
When the speedometer reads 60 mph (calibrated for 275/55R20), the actual speed with 285/55R20 is 60.8 mph — an error of +1.35%.
Is it safe to replace 275/55R20 with 285/55R20?
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.