TireSizesFinder

305/45R20 vs 275/45R21

275/45R21 is 0.06 in (1.6 mm) shorter than 305/45R20, a −0.20% diameter difference. With the speedometer set for 305/45R20, an indicated 60 mph is 59.9 mph on 275/45R21.

Is this a safe swap?

Diameter difference −0.20%: under 1%

By overall diameter alone, 275/45R21 is a near match for 305/45R20; 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: 305/45R20 (30.81 in) vs 275/45R21 (30.74 in)305/45R2030.81 in × 12.01 in275/45R2130.74 in × 10.83 in
Side profile and tread width, drawn to the same scale.
305/45R20 vs 275/45R21 differences
Measurement305/45R20275/45R21Difference% diff
Overall diameter30.81 in (782.5 mm)30.74 in (780.9 mm)−0.06 in (−1.6 mm)−0.20%
Sidewall height5.40 in (137.3 mm)4.87 in (123.8 mm)−0.53 in (−13.5 mm)−9.84%
Section width12.01 in (305.0 mm)10.83 in (275.0 mm)−1.18 in (−30.0 mm)−9.84%
Circumference96.78 in (2,458.3 mm)96.59 in (2,453.3 mm)−0.20 in (−5.0 mm)−0.20%
Revolutions per mile655656+1+0.20%
True speed with the speedometer calibrated for 305/45R20
IndicatedActual on 305/45R20Actual on 275/45R21Difference
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: 275/45R21 lowers the vehicle by 0.03 in (0.8 mm) at the axle (half the −1.6 mm diameter difference).

275/45R21 is 0.06 in shorter than 305/45R20 and 30 mm narrower, a 0.20% difference in overall diameter.

With the speedometer calibrated for 305/45R20, an indicated 60 mph is actually 59.9 mph on 275/45R21, and revolutions per mile change from 655 to 656.

275/45R21 mounts on a 21-inch wheel, a plus-1 fitment from the 20-inch wheel of 305/45R20.

Size pages

Other comparisons with 305/45R20

Other comparisons with 275/45R21

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