TireSizesFinder

255/40R20 vs 245/35R21

245/35R21 is 0.28 in (7.1 mm) shorter than 255/40R20, a −1.00% diameter difference. With the speedometer set for 255/40R20, an indicated 60 mph is 59.4 mph on 245/35R21.

Is this a safe swap?

Diameter difference −1.00%: under 1%

By overall diameter alone, 245/35R21 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 245/35R21 (27.75 in)255/40R2028.03 in × 10.04 in245/35R2127.75 in × 9.65 in
Side profile and tread width, drawn to the same scale.
255/40R20 vs 245/35R21 differences
Measurement255/40R20245/35R21Difference% diff
Overall diameter28.03 in (712.0 mm)27.75 in (704.9 mm)−0.28 in (−7.1 mm)−1.00%
Sidewall height4.02 in (102.0 mm)3.38 in (85.8 mm)−0.64 in (−16.2 mm)−15.93%
Section width10.04 in (255.0 mm)9.65 in (245.0 mm)−0.39 in (−10.0 mm)−3.92%
Circumference88.06 in (2,236.8 mm)87.19 in (2,214.5 mm)−0.88 in (−22.3 mm)−1.00%
Revolutions per mile719727+7+1.01%
True speed with the speedometer calibrated for 255/40R20
IndicatedActual on 255/40R20Actual on 245/35R21Difference
30 mph30.0 mph29.7 mph−0.3 mph
60 mph60.0 mph59.4 mph−0.6 mph
75 mph75.0 mph74.3 mph−0.7 mph

Ride-height change: 245/35R21 lowers the vehicle by 0.14 in (3.6 mm) at the axle (half the −7.1 mm diameter difference).

245/35R21 is 0.28 in shorter than 255/40R20 and 10 mm narrower, a 1.00% difference in overall diameter.

With the speedometer calibrated for 255/40R20, an indicated 60 mph is actually 59.4 mph on 245/35R21, and revolutions per mile change from 719 to 727.

245/35R21 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 245/35R21

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