TireSizesFinder

305/40R20 vs 275/40R21

275/40R21 is 0.06 in (1.4 mm) taller than 305/40R20, a +0.19% diameter difference. With the speedometer set for 305/40R20, an indicated 60 mph is 60.1 mph on 275/40R21.

Is this a safe swap?

Diameter difference +0.19%: under 1%

By overall diameter alone, 275/40R21 is a near match for 305/40R20; 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: 305/40R20 (29.61 in) vs 275/40R21 (29.66 in)305/40R2029.61 in × 12.01 in275/40R2129.66 in × 10.83 in
Side profile and tread width, drawn to the same scale.
305/40R20 vs 275/40R21 differences
Measurement305/40R20275/40R21Difference% diff
Overall diameter29.61 in (752.0 mm)29.66 in (753.4 mm)+0.06 in (+1.4 mm)+0.19%
Sidewall height4.80 in (122.0 mm)4.33 in (110.0 mm)−0.47 in (−12.0 mm)−9.84%
Section width12.01 in (305.0 mm)10.83 in (275.0 mm)−1.18 in (−30.0 mm)−9.84%
Circumference93.01 in (2,362.5 mm)93.18 in (2,366.9 mm)+0.17 in (+4.4 mm)+0.19%
Revolutions per mile681680−1−0.19%
True speed with the speedometer calibrated for 305/40R20
IndicatedActual on 305/40R20Actual on 275/40R21Difference
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: 275/40R21 raises the vehicle by 0.03 in (0.7 mm) at the axle (half the +1.4 mm diameter difference).

275/40R21 is 0.06 in taller than 305/40R20 and 30 mm narrower, a 0.19% difference in overall diameter.

With the speedometer calibrated for 305/40R20, an indicated 60 mph is actually 60.1 mph on 275/40R21, and revolutions per mile change from 681 to 680.

275/40R21 mounts on a 21-inch wheel, a plus-1 fitment from the 20-inch wheel of 305/40R20.

Size pages

Other comparisons with 305/40R20

Other comparisons with 275/40R21

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