TireSizesFinder

255/45R20 vs 235/45R21

235/45R21 is 0.29 in (7.4 mm) taller than 255/45R20, a +1.00% diameter difference. With the speedometer set for 255/45R20, an indicated 60 mph is 60.6 mph on 235/45R21.

Is this a safe swap?

Diameter difference +1.00%: within the ±3% guideline

By overall diameter alone, 235/45R21 falls inside the ±3% range commonly used as a guideline when replacing 255/45R20. This check uses diameter only. It does not assess load index, speed rating, wheel width or clearance.

To-scale drawing: 255/45R20 (29.04 in) vs 235/45R21 (29.33 in)255/45R2029.04 in × 10.04 in235/45R2129.33 in × 9.25 in
Side profile and tread width, drawn to the same scale.
255/45R20 vs 235/45R21 differences
Measurement255/45R20235/45R21Difference% diff
Overall diameter29.04 in (737.5 mm)29.33 in (744.9 mm)+0.29 in (+7.4 mm)+1.00%
Sidewall height4.52 in (114.8 mm)4.16 in (105.8 mm)−0.35 in (−9.0 mm)−7.84%
Section width10.04 in (255.0 mm)9.25 in (235.0 mm)−0.79 in (−20.0 mm)−7.84%
Circumference91.22 in (2,316.9 mm)92.13 in (2,340.2 mm)+0.92 in (+23.2 mm)+1.00%
Revolutions per mile695688−7−0.99%
True speed with the speedometer calibrated for 255/45R20
IndicatedActual on 255/45R20Actual on 235/45R21Difference
30 mph30.0 mph30.3 mph+0.3 mph
60 mph60.0 mph60.6 mph+0.6 mph
75 mph75.0 mph75.8 mph+0.8 mph

Ride-height change: 235/45R21 raises the vehicle by 0.15 in (3.7 mm) at the axle (half the +7.4 mm diameter difference).

235/45R21 is 0.29 in taller than 255/45R20 and 20 mm narrower, a 1.00% difference in overall diameter.

With the speedometer calibrated for 255/45R20, an indicated 60 mph is actually 60.6 mph on 235/45R21, and revolutions per mile change from 695 to 688.

235/45R21 mounts on a 21-inch wheel, a plus-1 fitment from the 20-inch wheel of 255/45R20.

Size pages

Other comparisons with 255/45R20

Other comparisons with 235/45R21

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