TireSizesFinder

235/45R21 vs 275/40R21

275/40R21 is 0.33 in (8.5 mm) taller than 235/45R21, a +1.14% diameter difference. With the speedometer set for 235/45R21, an indicated 60 mph is 60.7 mph on 275/40R21.

Is this a safe swap?

Diameter difference +1.14%: within the ±3% guideline

By overall diameter alone, 275/40R21 falls inside the ±3% range commonly used as a guideline when replacing 235/45R21. This check uses diameter only. It does not assess load index, speed rating, wheel width or clearance.

To-scale drawing: 235/45R21 (29.33 in) vs 275/40R21 (29.66 in)235/45R2129.33 in × 9.25 in275/40R2129.66 in × 10.83 in
Side profile and tread width, drawn to the same scale.
235/45R21 vs 275/40R21 differences
Measurement235/45R21275/40R21Difference% diff
Overall diameter29.33 in (744.9 mm)29.66 in (753.4 mm)+0.33 in (+8.5 mm)+1.14%
Sidewall height4.16 in (105.8 mm)4.33 in (110.0 mm)+0.17 in (+4.3 mm)+4.02%
Section width9.25 in (235.0 mm)10.83 in (275.0 mm)+1.57 in (+40.0 mm)+17.02%
Circumference92.13 in (2,340.2 mm)93.18 in (2,366.9 mm)+1.05 in (+26.7 mm)+1.14%
Revolutions per mile688680−8−1.13%
True speed with the speedometer calibrated for 235/45R21
IndicatedActual on 235/45R21Actual on 275/40R21Difference
30 mph30.0 mph30.3 mph+0.3 mph
60 mph60.0 mph60.7 mph+0.7 mph
75 mph75.0 mph75.9 mph+0.9 mph

Ride-height change: 275/40R21 raises the vehicle by 0.17 in (4.3 mm) at the axle (half the +8.5 mm diameter difference).

275/40R21 is 0.33 in taller than 235/45R21 and 40 mm wider, a 1.14% difference in overall diameter.

With the speedometer calibrated for 235/45R21, an indicated 60 mph is actually 60.7 mph on 275/40R21, and revolutions per mile change from 688 to 680.

Both sizes mount on 21-inch wheels.

Size pages

Other comparisons with 235/45R21

Other comparisons with 275/40R21

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