TireSizesFinder

305/40R20 vs 245/45R21

245/45R21 is 0.07 in (1.9 mm) taller than 305/40R20, a +0.25% diameter difference. With the speedometer set for 305/40R20, an indicated 60 mph is 60.2 mph on 245/45R21.

Is this a safe swap?

Diameter difference +0.25%: under 1%

By overall diameter alone, 245/45R21 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 245/45R21 (29.68 in)305/40R2029.61 in × 12.01 in245/45R2129.68 in × 9.65 in
Side profile and tread width, drawn to the same scale.
305/40R20 vs 245/45R21 differences
Measurement305/40R20245/45R21Difference% diff
Overall diameter29.61 in (752.0 mm)29.68 in (753.9 mm)+0.07 in (+1.9 mm)+0.25%
Sidewall height4.80 in (122.0 mm)4.34 in (110.3 mm)−0.46 in (−11.7 mm)−9.63%
Section width12.01 in (305.0 mm)9.65 in (245.0 mm)−2.36 in (−60.0 mm)−19.67%
Circumference93.01 in (2,362.5 mm)93.25 in (2,368.4 mm)+0.24 in (+6.0 mm)+0.25%
Revolutions per mile681679−2−0.25%
True speed with the speedometer calibrated for 305/40R20
IndicatedActual on 305/40R20Actual on 245/45R21Difference
30 mph30.0 mph30.1 mph+0.1 mph
60 mph60.0 mph60.2 mph+0.2 mph
75 mph75.0 mph75.2 mph+0.2 mph

Ride-height change: 245/45R21 raises the vehicle by 0.04 in (0.9 mm) at the axle (half the +1.9 mm diameter difference).

245/45R21 is 0.07 in taller than 305/40R20 and 60 mm narrower, a 0.25% difference in overall diameter.

With the speedometer calibrated for 305/40R20, an indicated 60 mph is actually 60.2 mph on 245/45R21, and revolutions per mile change from 681 to 679.

245/45R21 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 245/45R21

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