245/35R20 vs 255/30R20
Diameter -2.72% · Speedo at 60 mph reads 58.4 mph actual · Ground clearance -9.3 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 245/35R20) reads 60 mph, the actual speed with 255/30R20 mounted is:
Speedometer error: -2.72% · Odometer reads low by 2.72%
Full comparison
| Measurement | 245/35R20 | 255/30R20 | Delta |
|---|---|---|---|
| Overall diameter | 26.75" | 26.02" | -2.72% |
| Section width | 245 mm | 255 mm | +10 mm |
| Sidewall height | 85.75 mm | 76.5 mm | -9.25 mm |
| Rim diameter | 20" | 20" | +0" |
| Circumference | 2135 mm | 2077 mm | -58 mm |
| Revs / mile | 754 | 775 | 21 |
| Ground clearance change | -9.3 mm | ||
Questions about this swap
What is the diameter difference between 245/35R20 and 255/30R20?
255/30R20 is 2.72% smaller in overall diameter than 245/35R20 — a difference of 18.5 mm (0.73 inches).
How does 255/30R20 affect the speedometer if the car was calibrated for 245/35R20?
When the speedometer reads 60 mph (calibrated for 245/35R20), the actual speed with 255/30R20 is 58.4 mph — an error of -2.72%.
Is it safe to replace 245/35R20 with 255/30R20?
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.