245/40R18 vs 225/30R20

225/30R20 is 0.40 in (10.2 mm) shorter than 245/40R18, a −1.56% diameter difference. With the speedometer set for 245/40R18, an indicated 60 mph is 59.1 mph on 225/30R20.

Is this a safe swap?

Diameter difference −1.56%: within the ±3% guideline

By overall diameter alone, 225/30R20 falls inside the ±3% range commonly used as a guideline when replacing 245/40R18. 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.

245/40R18 vs 225/30R20 differences
Measurement	245/40R18	225/30R20	Difference	% diff
Overall diameter	25.72 in (653.2 mm)	25.31 in (643.0 mm)	−0.40 in (−10.2 mm)	−1.56%
Sidewall height	3.86 in (98.0 mm)	2.66 in (67.5 mm)	−1.20 in (−30.5 mm)	−31.12%
Section width	9.65 in (245.0 mm)	8.86 in (225.0 mm)	−0.79 in (−20.0 mm)	−8.16%
Circumference	80.79 in (2,052.1 mm)	79.53 in (2,020.0 mm)	−1.26 in (−32.0 mm)	−1.56%
Revolutions per mile	784	797	+12	+1.59%

True speed with the speedometer calibrated for 245/40R18
Indicated	Actual on 245/40R18	Actual on 225/30R20	Difference
30 mph	30.0 mph	29.5 mph	−0.5 mph
60 mph	60.0 mph	59.1 mph	−0.9 mph
75 mph	75.0 mph	73.8 mph	−1.2 mph

Ride-height change: 225/30R20 lowers the vehicle by 0.20 in (5.1 mm) at the axle (half the −10.2 mm diameter difference).

225/30R20 is 0.40 in shorter than 245/40R18 and 20 mm narrower, a 1.56% difference in overall diameter.

With the speedometer calibrated for 245/40R18, an indicated 60 mph is actually 59.1 mph on 225/30R20, and revolutions per mile change from 784 to 797.

225/30R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 245/40R18.

Size pages

Other comparisons with 245/40R18

Other comparisons with 225/30R20

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