TireSizesFinder

275/40R20 vs 325/30R21

325/30R21 is 0.02 in (0.4 mm) taller than 275/40R20, a +0.05% diameter difference. With the speedometer set for 275/40R20, an indicated 60 mph is 60.0 mph on 325/30R21.

Is this a safe swap?

Diameter difference +0.05%: under 1%

By overall diameter alone, 325/30R21 is a near match for 275/40R20; 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: 275/40R20 (28.66 in) vs 325/30R21 (28.68 in)275/40R2028.66 in × 10.83 in325/30R2128.68 in × 12.80 in
Side profile and tread width, drawn to the same scale.
275/40R20 vs 325/30R21 differences
Measurement275/40R20325/30R21Difference% diff
Overall diameter28.66 in (728.0 mm)28.68 in (728.4 mm)+0.02 in (+0.4 mm)+0.05%
Sidewall height4.33 in (110.0 mm)3.84 in (97.5 mm)−0.49 in (−12.5 mm)−11.36%
Section width10.83 in (275.0 mm)12.80 in (325.0 mm)+1.97 in (+50.0 mm)+18.18%
Circumference90.04 in (2,287.1 mm)90.09 in (2,288.3 mm)+0.05 in (+1.3 mm)+0.05%
Revolutions per mile7047030−0.05%
True speed with the speedometer calibrated for 275/40R20
IndicatedActual on 275/40R20Actual on 325/30R21Difference
30 mph30.0 mph30.0 mph0.0 mph
60 mph60.0 mph60.0 mph0.0 mph
75 mph75.0 mph75.0 mph0.0 mph

Ride-height change: 325/30R21 raises the vehicle by 0.01 in (0.2 mm) at the axle (half the +0.4 mm diameter difference).

325/30R21 is 0.02 in taller than 275/40R20 and 50 mm wider, a 0.05% difference in overall diameter.

With the speedometer calibrated for 275/40R20, an indicated 60 mph is actually 60.0 mph on 325/30R21, and revolutions per mile change from 704 to 703.

325/30R21 mounts on a 21-inch wheel, a plus-1 fitment from the 20-inch wheel of 275/40R20.

Size pages

Other comparisons with 275/40R20

Other comparisons with 325/30R21

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