TireSizesFinder

275/45R20 vs 275/40R21

275/40R21 is 0.08 in (2.1 mm) shorter than 275/45R20, a −0.28% diameter difference. With the speedometer set for 275/45R20, an indicated 60 mph is 59.8 mph on 275/40R21.

Is this a safe swap?

Diameter difference −0.28%: under 1%

By overall diameter alone, 275/40R21 is a near match for 275/45R20; 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: 275/45R20 (29.74 in) vs 275/40R21 (29.66 in)275/45R2029.74 in × 10.83 in275/40R2129.66 in × 10.83 in
Side profile and tread width, drawn to the same scale.
275/45R20 vs 275/40R21 differences
Measurement275/45R20275/40R21Difference% diff
Overall diameter29.74 in (755.5 mm)29.66 in (753.4 mm)−0.08 in (−2.1 mm)−0.28%
Sidewall height4.87 in (123.8 mm)4.33 in (110.0 mm)−0.54 in (−13.7 mm)−11.11%
Section width10.83 in (275.0 mm)10.83 in (275.0 mm)0.00 in (0.0 mm)0.00%
Circumference93.44 in (2,373.5 mm)93.18 in (2,366.9 mm)−0.26 in (−6.6 mm)−0.28%
Revolutions per mile678680+2+0.28%
True speed with the speedometer calibrated for 275/45R20
IndicatedActual on 275/45R20Actual on 275/40R21Difference
30 mph30.0 mph29.9 mph−0.1 mph
60 mph60.0 mph59.8 mph−0.2 mph
75 mph75.0 mph74.8 mph−0.2 mph

Ride-height change: 275/40R21 lowers the vehicle by 0.04 in (1.1 mm) at the axle (half the −2.1 mm diameter difference).

275/40R21 is 0.08 in shorter than 275/45R20 and the same width, a 0.28% difference in overall diameter.

With the speedometer calibrated for 275/45R20, an indicated 60 mph is actually 59.8 mph on 275/40R21, and revolutions per mile change from 678 to 680.

275/40R21 mounts on a 21-inch wheel, a plus-1 fitment from the 20-inch wheel of 275/45R20.

Size pages

Other comparisons with 275/45R20

Other comparisons with 275/40R21

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