TireSizesFinder

255/40R21 vs 235/45R21

235/45R21 is 0.30 in (7.5 mm) taller than 255/40R21, a +1.02% diameter difference. With the speedometer set for 255/40R21, an indicated 60 mph is 60.6 mph on 235/45R21.

Is this a safe swap?

Diameter difference +1.02%: within the ±3% guideline

By overall diameter alone, 235/45R21 falls inside the ±3% range commonly used as a guideline when replacing 255/40R21. This check uses diameter only. It does not assess load index, speed rating, wheel width or clearance.

To-scale drawing: 255/40R21 (29.03 in) vs 235/45R21 (29.33 in)255/40R2129.03 in × 10.04 in235/45R2129.33 in × 9.25 in
Side profile and tread width, drawn to the same scale.
255/40R21 vs 235/45R21 differences
Measurement255/40R21235/45R21Difference% diff
Overall diameter29.03 in (737.4 mm)29.33 in (744.9 mm)+0.30 in (+7.5 mm)+1.02%
Sidewall height4.02 in (102.0 mm)4.16 in (105.8 mm)+0.15 in (+3.8 mm)+3.68%
Section width10.04 in (255.0 mm)9.25 in (235.0 mm)−0.79 in (−20.0 mm)−7.84%
Circumference91.21 in (2,316.6 mm)92.13 in (2,340.2 mm)+0.93 in (+23.6 mm)+1.02%
Revolutions per mile695688−7−1.01%
True speed with the speedometer calibrated for 255/40R21
IndicatedActual on 255/40R21Actual 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 (3.8 mm) at the axle (half the +7.5 mm diameter difference).

235/45R21 is 0.30 in taller than 255/40R21 and 20 mm narrower, a 1.02% difference in overall diameter.

With the speedometer calibrated for 255/40R21, an indicated 60 mph is actually 60.6 mph on 235/45R21, and revolutions per mile change from 695 to 688.

Both sizes mount on 21-inch wheels.

Size pages

Other comparisons with 255/40R21

Other comparisons with 235/45R21

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