245/45R18 vs 285/30R20
Comparison of overall diameter, sidewall, speedometer error, and ride-height change between these two tire sizes.
245/45R18
- Section width
- 245 mm
- Aspect ratio
- 45%
- Rim
- 18"
- Sidewall
- 110.3 mm
- Overall diameter
- 677.7 mm
- Revs / mile
- 756
285/30R20
- Section width
- 285 mm
- Aspect ratio
- 30%
- Rim
- 20"
- Sidewall
- 85.5 mm
- Overall diameter
- 679.0 mm
- Revs / mile
- 754
Difference
- Overall diameter
- +1.3 mm (+0.19%)
- Sidewall
- -24.8 mm
- Tread width
- +40 mm
- Ride height (axle)
- +0.6 mm
- Speedometer error
- +0.19%
- At indicated 60 mph
- actual 60.12 mph
Will 285/30R20 fit instead of 245/45R18?
The overall diameter changes by 0.19%. 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).