TireSizesFinder

275/45R20 vs 255/45R21

255/45R21 is 0.29 in (7.4 mm) taller than 275/45R20, a +0.98% diameter difference. With the speedometer set for 275/45R20, an indicated 60 mph is 60.6 mph on 255/45R21.

Is this a safe swap?

Diameter difference +0.98%: under 1%

By overall diameter alone, 255/45R21 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 255/45R21 (30.04 in)275/45R2029.74 in × 10.83 in255/45R2130.04 in × 10.04 in
Side profile and tread width, drawn to the same scale.
275/45R20 vs 255/45R21 differences
Measurement275/45R20255/45R21Difference% diff
Overall diameter29.74 in (755.5 mm)30.04 in (762.9 mm)+0.29 in (+7.4 mm)+0.98%
Sidewall height4.87 in (123.8 mm)4.52 in (114.8 mm)−0.35 in (−9.0 mm)−7.27%
Section width10.83 in (275.0 mm)10.04 in (255.0 mm)−0.79 in (−20.0 mm)−7.27%
Circumference93.44 in (2,373.5 mm)94.36 in (2,396.7 mm)+0.92 in (+23.2 mm)+0.98%
Revolutions per mile678671−7−0.97%
True speed with the speedometer calibrated for 275/45R20
IndicatedActual on 275/45R20Actual on 255/45R21Difference
30 mph30.0 mph30.3 mph+0.3 mph
60 mph60.0 mph60.6 mph+0.6 mph
75 mph75.0 mph75.7 mph+0.7 mph

Ride-height change: 255/45R21 raises the vehicle by 0.15 in (3.7 mm) at the axle (half the +7.4 mm diameter difference).

255/45R21 is 0.29 in taller than 275/45R20 and 20 mm narrower, a 0.98% difference in overall diameter.

With the speedometer calibrated for 275/45R20, an indicated 60 mph is actually 60.6 mph on 255/45R21, and revolutions per mile change from 678 to 671.

255/45R21 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 255/45R21

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