215/45R18 vs 225/30R20

225/30R20 is 0.30 in (7.7 mm) shorter than 215/45R18, a −1.18% diameter difference. With the speedometer set for 215/45R18, an indicated 60 mph is 59.3 mph on 225/30R20.

Is this a safe swap?

Diameter difference −1.18%: within the ±3% guideline

By overall diameter alone, 225/30R20 falls inside the ±3% range commonly used as a guideline when replacing 215/45R18. This check uses diameter only. It does not assess load index, speed rating, wheel width or clearance.

Side profile and tread width, drawn to the same scale.

215/45R18 vs 225/30R20 differences
Measurement	215/45R18	225/30R20	Difference	% diff
Overall diameter	25.62 in (650.7 mm)	25.31 in (643.0 mm)	−0.30 in (−7.7 mm)	−1.18%
Sidewall height	3.81 in (96.8 mm)	2.66 in (67.5 mm)	−1.15 in (−29.2 mm)	−30.23%
Section width	8.46 in (215.0 mm)	8.86 in (225.0 mm)	+0.39 in (+10.0 mm)	+4.65%
Circumference	80.48 in (2,044.2 mm)	79.53 in (2,020.0 mm)	−0.95 in (−24.2 mm)	−1.18%
Revolutions per mile	787	797	+9	+1.20%

True speed with the speedometer calibrated for 215/45R18
Indicated	Actual on 215/45R18	Actual on 225/30R20	Difference
30 mph	30.0 mph	29.6 mph	−0.4 mph
60 mph	60.0 mph	59.3 mph	−0.7 mph
75 mph	75.0 mph	74.1 mph	−0.9 mph

Ride-height change: 225/30R20 lowers the vehicle by 0.15 in (3.9 mm) at the axle (half the −7.7 mm diameter difference).

225/30R20 is 0.30 in shorter than 215/45R18 and 10 mm wider, a 1.18% difference in overall diameter.

With the speedometer calibrated for 215/45R18, an indicated 60 mph is actually 59.3 mph on 225/30R20, and revolutions per mile change from 787 to 797.

225/30R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 215/45R18.

Size pages

Other comparisons with 215/45R18

Other comparisons with 225/30R20

Source: Calculated from standard tire geometry formulas. Verified 2026-09-27. How these numbers are calculated.