TireSizesFinder

275/45R20 vs 245/45R21

245/45R21 is 0.06 in (1.6 mm) shorter than 275/45R20, a −0.21% diameter difference. With the speedometer set for 275/45R20, an indicated 60 mph is 59.9 mph on 245/45R21.

Is this a safe swap?

Diameter difference −0.21%: under 1%

By overall diameter alone, 245/45R21 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 245/45R21 (29.68 in)275/45R2029.74 in × 10.83 in245/45R2129.68 in × 9.65 in
Side profile and tread width, drawn to the same scale.
275/45R20 vs 245/45R21 differences
Measurement275/45R20245/45R21Difference% diff
Overall diameter29.74 in (755.5 mm)29.68 in (753.9 mm)−0.06 in (−1.6 mm)−0.21%
Sidewall height4.87 in (123.8 mm)4.34 in (110.3 mm)−0.53 in (−13.5 mm)−10.91%
Section width10.83 in (275.0 mm)9.65 in (245.0 mm)−1.18 in (−30.0 mm)−10.91%
Circumference93.44 in (2,373.5 mm)93.25 in (2,368.4 mm)−0.20 in (−5.0 mm)−0.21%
Revolutions per mile678679+1+0.21%
True speed with the speedometer calibrated for 275/45R20
IndicatedActual on 275/45R20Actual on 245/45R21Difference
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: 245/45R21 lowers the vehicle by 0.03 in (0.8 mm) at the axle (half the −1.6 mm diameter difference).

245/45R21 is 0.06 in shorter than 275/45R20 and 30 mm narrower, a 0.21% difference in overall diameter.

With the speedometer calibrated for 275/45R20, an indicated 60 mph is actually 59.9 mph on 245/45R21, and revolutions per mile change from 678 to 679.

245/45R21 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 245/45R21

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