TireSizesFinder

275/40R20 vs 245/40R21

245/40R21 is 0.06 in (1.4 mm) taller than 275/40R20, a +0.19% diameter difference. With the speedometer set for 275/40R20, an indicated 60 mph is 60.1 mph on 245/40R21.

Is this a safe swap?

Diameter difference +0.19%: under 1%

By overall diameter alone, 245/40R21 is a near match for 275/40R20; 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: 275/40R20 (28.66 in) vs 245/40R21 (28.72 in)275/40R2028.66 in × 10.83 in245/40R2128.72 in × 9.65 in
Side profile and tread width, drawn to the same scale.
275/40R20 vs 245/40R21 differences
Measurement275/40R20245/40R21Difference% diff
Overall diameter28.66 in (728.0 mm)28.72 in (729.4 mm)+0.06 in (+1.4 mm)+0.19%
Sidewall height4.33 in (110.0 mm)3.86 in (98.0 mm)−0.47 in (−12.0 mm)−10.91%
Section width10.83 in (275.0 mm)9.65 in (245.0 mm)−1.18 in (−30.0 mm)−10.91%
Circumference90.04 in (2,287.1 mm)90.22 in (2,291.5 mm)+0.17 in (+4.4 mm)+0.19%
Revolutions per mile704702−1−0.19%
True speed with the speedometer calibrated for 275/40R20
IndicatedActual on 275/40R20Actual on 245/40R21Difference
30 mph30.0 mph30.1 mph+0.1 mph
60 mph60.0 mph60.1 mph+0.1 mph
75 mph75.0 mph75.1 mph+0.1 mph

Ride-height change: 245/40R21 raises the vehicle by 0.03 in (0.7 mm) at the axle (half the +1.4 mm diameter difference).

245/40R21 is 0.06 in taller than 275/40R20 and 30 mm narrower, a 0.19% difference in overall diameter.

With the speedometer calibrated for 275/40R20, an indicated 60 mph is actually 60.1 mph on 245/40R21, and revolutions per mile change from 704 to 702.

245/40R21 mounts on a 21-inch wheel, a plus-1 fitment from the 20-inch wheel of 275/40R20.

Size pages

Other comparisons with 275/40R20

Other comparisons with 245/40R21

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