TireSizesFinder

235/55R19 vs 255/40R21

255/40R21 is 0.15 in (3.7 mm) shorter than 235/55R19, a −0.50% diameter difference. With the speedometer set for 235/55R19, an indicated 60 mph is 59.7 mph on 255/40R21.

Is this a safe swap?

Diameter difference −0.50%: under 1%

By overall diameter alone, 255/40R21 is a near match for 235/55R19; 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: 235/55R19 (29.18 in) vs 255/40R21 (29.03 in)235/55R1929.18 in × 9.25 in255/40R2129.03 in × 10.04 in
Side profile and tread width, drawn to the same scale.
235/55R19 vs 255/40R21 differences
Measurement235/55R19255/40R21Difference% diff
Overall diameter29.18 in (741.1 mm)29.03 in (737.4 mm)−0.15 in (−3.7 mm)−0.50%
Sidewall height5.09 in (129.3 mm)4.02 in (102.0 mm)−1.07 in (−27.2 mm)−21.08%
Section width9.25 in (235.0 mm)10.04 in (255.0 mm)+0.79 in (+20.0 mm)+8.51%
Circumference91.66 in (2,328.2 mm)91.21 in (2,316.6 mm)−0.46 in (−11.6 mm)−0.50%
Revolutions per mile691695+3+0.50%
True speed with the speedometer calibrated for 235/55R19
IndicatedActual on 235/55R19Actual on 255/40R21Difference
30 mph30.0 mph29.9 mph−0.1 mph
60 mph60.0 mph59.7 mph−0.3 mph
75 mph75.0 mph74.6 mph−0.4 mph

Ride-height change: 255/40R21 lowers the vehicle by 0.07 in (1.8 mm) at the axle (half the −3.7 mm diameter difference).

255/40R21 is 0.15 in shorter than 235/55R19 and 20 mm wider, a 0.50% difference in overall diameter.

With the speedometer calibrated for 235/55R19, an indicated 60 mph is actually 59.7 mph on 255/40R21, and revolutions per mile change from 691 to 695.

255/40R21 mounts on a 21-inch wheel, a plus-2 fitment from the 19-inch wheel of 235/55R19.

Size pages

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