TireSizesFinder

255/40R21 vs 275/40R21

275/40R21 is 0.63 in (16.0 mm) taller than 255/40R21, a +2.17% diameter difference. With the speedometer set for 255/40R21, an indicated 60 mph is 61.3 mph on 275/40R21.

Is this a safe swap?

Diameter difference +2.17%: within the ±3% guideline

By overall diameter alone, 275/40R21 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 275/40R21 (29.66 in)255/40R2129.03 in × 10.04 in275/40R2129.66 in × 10.83 in
Side profile and tread width, drawn to the same scale.
255/40R21 vs 275/40R21 differences
Measurement255/40R21275/40R21Difference% diff
Overall diameter29.03 in (737.4 mm)29.66 in (753.4 mm)+0.63 in (+16.0 mm)+2.17%
Sidewall height4.02 in (102.0 mm)4.33 in (110.0 mm)+0.31 in (+8.0 mm)+7.84%
Section width10.04 in (255.0 mm)10.83 in (275.0 mm)+0.79 in (+20.0 mm)+7.84%
Circumference91.21 in (2,316.6 mm)93.18 in (2,366.9 mm)+1.98 in (+50.3 mm)+2.17%
Revolutions per mile695680−15−2.12%
True speed with the speedometer calibrated for 255/40R21
IndicatedActual on 255/40R21Actual on 275/40R21Difference
30 mph30.0 mph30.7 mph+0.7 mph
60 mph60.0 mph61.3 mph+1.3 mph
75 mph75.0 mph76.6 mph+1.6 mph

Ride-height change: 275/40R21 raises the vehicle by 0.31 in (8.0 mm) at the axle (half the +16.0 mm diameter difference).

275/40R21 is 0.63 in taller than 255/40R21 and 20 mm wider, a 2.17% difference in overall diameter.

With the speedometer calibrated for 255/40R21, an indicated 60 mph is actually 61.3 mph on 275/40R21, and revolutions per mile change from 695 to 680.

Both sizes mount on 21-inch wheels.

Size pages

Other comparisons with 255/40R21

Other comparisons with 275/40R21

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