TireSizesFinder

235/60R18 vs 255/45R20

255/45R20 is 0.07 in (1.7 mm) shorter than 235/60R18, a −0.23% diameter difference. With the speedometer set for 235/60R18, an indicated 60 mph is 59.9 mph on 255/45R20.

Is this a safe swap?

Diameter difference −0.23%: under 1%

By overall diameter alone, 255/45R20 is a near match for 235/60R18; 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: 235/60R18 (29.10 in) vs 255/45R20 (29.04 in)235/60R1829.10 in × 9.25 in255/45R2029.04 in × 10.04 in
Side profile and tread width, drawn to the same scale.
235/60R18 vs 255/45R20 differences
Measurement235/60R18255/45R20Difference% diff
Overall diameter29.10 in (739.2 mm)29.04 in (737.5 mm)−0.07 in (−1.7 mm)−0.23%
Sidewall height5.55 in (141.0 mm)4.52 in (114.8 mm)−1.03 in (−26.2 mm)−18.62%
Section width9.25 in (235.0 mm)10.04 in (255.0 mm)+0.79 in (+20.0 mm)+8.51%
Circumference91.43 in (2,322.3 mm)91.22 in (2,316.9 mm)−0.21 in (−5.3 mm)−0.23%
Revolutions per mile693695+2+0.23%
True speed with the speedometer calibrated for 235/60R18
IndicatedActual on 235/60R18Actual on 255/45R20Difference
30 mph30.0 mph29.9 mph−0.1 mph
60 mph60.0 mph59.9 mph−0.1 mph
75 mph75.0 mph74.8 mph−0.2 mph

Ride-height change: 255/45R20 lowers the vehicle by 0.03 in (0.9 mm) at the axle (half the −1.7 mm diameter difference).

255/45R20 is 0.07 in shorter than 235/60R18 and 20 mm wider, a 0.23% difference in overall diameter.

With the speedometer calibrated for 235/60R18, an indicated 60 mph is actually 59.9 mph on 255/45R20, and revolutions per mile change from 693 to 695.

255/45R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 235/60R18.

Size pages

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Other comparisons with 255/45R20

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