TireSizesFinder

235/40R18 vs 225/30R20

225/30R20 is 0.09 in (2.2 mm) shorter than 235/40R18, a −0.34% diameter difference. With the speedometer set for 235/40R18, an indicated 60 mph is 59.8 mph on 225/30R20.

Is this a safe swap?

Diameter difference −0.34%: under 1%

By overall diameter alone, 225/30R20 is a near match for 235/40R18; the speedometer changes by less than 1%. This check uses diameter only. It does not assess load index, speed rating, wheel width or clearance.

To-scale drawing: 235/40R18 (25.40 in) vs 225/30R20 (25.31 in)235/40R1825.40 in × 9.25 in225/30R2025.31 in × 8.86 in
Side profile and tread width, drawn to the same scale.
235/40R18 vs 225/30R20 differences
Measurement235/40R18225/30R20Difference% diff
Overall diameter25.40 in (645.2 mm)25.31 in (643.0 mm)−0.09 in (−2.2 mm)−0.34%
Sidewall height3.70 in (94.0 mm)2.66 in (67.5 mm)−1.04 in (−26.5 mm)−28.19%
Section width9.25 in (235.0 mm)8.86 in (225.0 mm)−0.39 in (−10.0 mm)−4.26%
Circumference79.80 in (2,027.0 mm)79.53 in (2,020.0 mm)−0.27 in (−6.9 mm)−0.34%
Revolutions per mile794797+3+0.34%
True speed with the speedometer calibrated for 235/40R18
IndicatedActual on 235/40R18Actual on 225/30R20Difference
30 mph30.0 mph29.9 mph−0.1 mph
60 mph60.0 mph59.8 mph−0.2 mph
75 mph75.0 mph74.7 mph−0.3 mph

Ride-height change: 225/30R20 lowers the vehicle by 0.04 in (1.1 mm) at the axle (half the −2.2 mm diameter difference).

225/30R20 is 0.09 in shorter than 235/40R18 and 10 mm narrower, a 0.34% difference in overall diameter.

With the speedometer calibrated for 235/40R18, an indicated 60 mph is actually 59.8 mph on 225/30R20, and revolutions per mile change from 794 to 797.

225/30R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 235/40R18.

Size pages

Other comparisons with 235/40R18

Other comparisons with 225/30R20

Source: Calculated from standard tire geometry formulas. Verified . How these numbers are calculated.