TireSizesFinder

255/40R20 vs 295/30R21

295/30R21 is 0.06 in (1.6 mm) shorter than 255/40R20, a −0.22% diameter difference. With the speedometer set for 255/40R20, an indicated 60 mph is 59.9 mph on 295/30R21.

Is this a safe swap?

Diameter difference −0.22%: under 1%

By overall diameter alone, 295/30R21 is a near match for 255/40R20; 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: 255/40R20 (28.03 in) vs 295/30R21 (27.97 in)255/40R2028.03 in × 10.04 in295/30R2127.97 in × 11.61 in
Side profile and tread width, drawn to the same scale.
255/40R20 vs 295/30R21 differences
Measurement255/40R20295/30R21Difference% diff
Overall diameter28.03 in (712.0 mm)27.97 in (710.4 mm)−0.06 in (−1.6 mm)−0.22%
Sidewall height4.02 in (102.0 mm)3.48 in (88.5 mm)−0.53 in (−13.5 mm)−13.24%
Section width10.04 in (255.0 mm)11.61 in (295.0 mm)+1.57 in (+40.0 mm)+15.69%
Circumference88.06 in (2,236.8 mm)87.87 in (2,231.8 mm)−0.20 in (−5.0 mm)−0.22%
Revolutions per mile719721+2+0.23%
True speed with the speedometer calibrated for 255/40R20
IndicatedActual on 255/40R20Actual on 295/30R21Difference
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: 295/30R21 lowers the vehicle by 0.03 in (0.8 mm) at the axle (half the −1.6 mm diameter difference).

295/30R21 is 0.06 in shorter than 255/40R20 and 40 mm wider, a 0.22% difference in overall diameter.

With the speedometer calibrated for 255/40R20, an indicated 60 mph is actually 59.9 mph on 295/30R21, and revolutions per mile change from 719 to 721.

295/30R21 mounts on a 21-inch wheel, a plus-1 fitment from the 20-inch wheel of 255/40R20.

Size pages

Other comparisons with 255/40R20

Other comparisons with 295/30R21

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