255/40R20 vs 255/45R20
Side-by-side comparison of overall diameter, sidewall height, speedometer error, and ride-height change.
Not recommended — Exceeds the 3% diameter limit — speedometer error and ABS/TPMS issues likely.
To scale
255/40R20
- Section width
- 255 mm
- Aspect ratio
- 40%
- Rim diameter
- 20"
- Sidewall
- 102.0 mm
- Overall diameter
- 712.0 mm
- Circumference
- 2237 mm
- Revs / mile
- 719
- Revs / km
- 447
255/45R20
- Section width
- 255 mm
- Aspect ratio
- 45%
- Rim diameter
- 20"
- Sidewall
- 114.8 mm
- Overall diameter
- 737.5 mm
- Circumference
- 2317 mm
- Revs / mile
- 695
- Revs / km
- 432
Difference
- Overall diameter
- +25.5 mm (+3.58%)
- Sidewall height
- +12.8 mm
- Tread width
- +0 mm
- Ride height (axle)
- +12.8 mm
- Speedometer error
- +3.58%
- At indicated 60 mph
- actual 62.1 mph
- At indicated 100 km/h
- actual 103.6 km/h
Will 255/45R20 fit instead of 255/40R20?
The overall diameter changes by 3.58%. Most manufacturers consider a swap acceptable when the difference stays under 3% — beyond that, you may see speedometer error, ABS/TPMS warnings, or wheel-well clearance problems. This page only covers the math; your vehicle's wheel arches, suspension geometry, and load rating still matter.
What the numbers mean
- Overall diameter — outside-to-outside height of the tire. Affects ride height, gearing, and speedometer calibration.
- Sidewall height — rubber from rim edge to tread. Lower sidewall gives sharper steering response but a harsher ride.
- Speedometer error — how far off your indicated speed will be after the swap. Positive means the speedo under-reads (actual speed is higher than shown).