TireSizesFinder

305/40R20 vs 235/45R21

235/45R21 is 0.28 in (7.1 mm) shorter than 305/40R20, a −0.94% diameter difference. With the speedometer set for 305/40R20, an indicated 60 mph is 59.4 mph on 235/45R21.

Is this a safe swap?

Diameter difference −0.94%: under 1%

By overall diameter alone, 235/45R21 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 235/45R21 (29.33 in)305/40R2029.61 in × 12.01 in235/45R2129.33 in × 9.25 in
Side profile and tread width, drawn to the same scale.
305/40R20 vs 235/45R21 differences
Measurement305/40R20235/45R21Difference% diff
Overall diameter29.61 in (752.0 mm)29.33 in (744.9 mm)−0.28 in (−7.1 mm)−0.94%
Sidewall height4.80 in (122.0 mm)4.16 in (105.8 mm)−0.64 in (−16.2 mm)−13.32%
Section width12.01 in (305.0 mm)9.25 in (235.0 mm)−2.76 in (−70.0 mm)−22.95%
Circumference93.01 in (2,362.5 mm)92.13 in (2,340.2 mm)−0.88 in (−22.3 mm)−0.94%
Revolutions per mile681688+6+0.95%
True speed with the speedometer calibrated for 305/40R20
IndicatedActual on 305/40R20Actual on 235/45R21Difference
30 mph30.0 mph29.7 mph−0.3 mph
60 mph60.0 mph59.4 mph−0.6 mph
75 mph75.0 mph74.3 mph−0.7 mph

Ride-height change: 235/45R21 lowers the vehicle by 0.14 in (3.6 mm) at the axle (half the −7.1 mm diameter difference).

235/45R21 is 0.28 in shorter than 305/40R20 and 70 mm narrower, a 0.94% difference in overall diameter.

With the speedometer calibrated for 305/40R20, an indicated 60 mph is actually 59.4 mph on 235/45R21, and revolutions per mile change from 681 to 688.

235/45R21 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 235/45R21

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