245/50R18 vs 245/40R20
Side-by-side comparison of overall diameter, sidewall height, speedometer error, and ride-height change.
Safe to swap — Diameter difference is under 1.5% — within normal tolerance.
To scale
245/50R18
- Section width
- 245 mm
- Aspect ratio
- 50%
- Rim diameter
- 18"
- Sidewall
- 122.5 mm
- Overall diameter
- 702.2 mm
- Circumference
- 2206 mm
- Revs / mile
- 730
- Revs / km
- 453
245/40R20
- Section width
- 245 mm
- Aspect ratio
- 40%
- Rim diameter
- 20"
- Sidewall
- 98.0 mm
- Overall diameter
- 704.0 mm
- Circumference
- 2212 mm
- Revs / mile
- 728
- Revs / km
- 452
Difference
- Overall diameter
- +1.8 mm (+0.26%)
- Sidewall height
- -24.5 mm
- Tread width
- +0 mm
- Ride height (axle)
- +0.9 mm
- Speedometer error
- +0.26%
- At indicated 60 mph
- actual 60.2 mph
- At indicated 100 km/h
- actual 100.3 km/h
Will 245/40R20 fit instead of 245/50R18?
The overall diameter changes by 0.26%. 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).