245/50R18 vs 245/40R20
Comparison of overall diameter, sidewall, speedometer error, and ride-height change between these two tire sizes.
245/50R18
- Section width
- 245 mm
- Aspect ratio
- 50%
- Rim
- 18"
- Sidewall
- 122.5 mm
- Overall diameter
- 702.2 mm
- Revs / mile
- 730
245/40R20
- Section width
- 245 mm
- Aspect ratio
- 40%
- Rim
- 20"
- Sidewall
- 98.0 mm
- Overall diameter
- 704.0 mm
- Revs / mile
- 728
Difference
- Overall diameter
- +1.8 mm (+0.26%)
- Sidewall
- -24.5 mm
- Tread width
- +0 mm
- Ride height (axle)
- +0.9 mm
- Speedometer error
- +0.26%
- At indicated 60 mph
- actual 60.15 mph
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 is the outside-to-outside height of the tire. Affects ride height, gearing, and speedometer.
- Sidewall is the height of the rubber from rim to tread. Lower sidewall = sharper steering, harsher ride.
- Speedometer error is the percentage your indicated speed will be off after the swap (positive = speedo reads lower than actual).