TireSizesFinder

225/55R18 vs 245/40R20

245/40R20 is 0.03 in (0.7 mm) shorter than 225/55R18, a −0.10% diameter difference. With the speedometer set for 225/55R18, an indicated 60 mph is 59.9 mph on 245/40R20.

Is this a safe swap?

Diameter difference −0.10%: under 1%

By overall diameter alone, 245/40R20 is a near match for 225/55R18; 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: 225/55R18 (27.74 in) vs 245/40R20 (27.72 in)225/55R1827.74 in × 8.86 in245/40R2027.72 in × 9.65 in
Side profile and tread width, drawn to the same scale.
225/55R18 vs 245/40R20 differences
Measurement225/55R18245/40R20Difference% diff
Overall diameter27.74 in (704.7 mm)27.72 in (704.0 mm)−0.03 in (−0.7 mm)−0.10%
Sidewall height4.87 in (123.8 mm)3.86 in (98.0 mm)−1.01 in (−25.7 mm)−20.81%
Section width8.86 in (225.0 mm)9.65 in (245.0 mm)+0.79 in (+20.0 mm)+8.89%
Circumference87.16 in (2,213.9 mm)87.07 in (2,211.7 mm)−0.09 in (−2.2 mm)−0.10%
Revolutions per mile727728+1+0.10%
True speed with the speedometer calibrated for 225/55R18
IndicatedActual on 225/55R18Actual on 245/40R20Difference
30 mph30.0 mph30.0 mph0.0 mph
60 mph60.0 mph59.9 mph−0.1 mph
75 mph75.0 mph74.9 mph−0.1 mph

Ride-height change: 245/40R20 lowers the vehicle by 0.01 in (0.4 mm) at the axle (half the −0.7 mm diameter difference).

245/40R20 is 0.03 in shorter than 225/55R18 and 20 mm wider, a 0.10% difference in overall diameter.

With the speedometer calibrated for 225/55R18, an indicated 60 mph is actually 59.9 mph on 245/40R20, and revolutions per mile change from 727 to 728.

245/40R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 225/55R18.

Size pages

Other comparisons with 225/55R18

Other comparisons with 245/40R20

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