275/35R21 vs 245/40R21

245/40R21 is 0.14 in (3.5 mm) taller than 275/35R21, a +0.48% diameter difference. With the speedometer set for 275/35R21, an indicated 60 mph is 60.3 mph on 245/40R21.

Is this a safe swap?

Diameter difference +0.48%: under 1%

By overall diameter alone, 245/40R21 is a near match for 275/35R21; the speedometer changes by less than 1%. This check uses diameter only. It does not assess load index, speed rating, wheel width or clearance.

Side profile and tread width, drawn to the same scale.

275/35R21 vs 245/40R21 differences
Measurement	275/35R21	245/40R21	Difference	% diff
Overall diameter	28.58 in (725.9 mm)	28.72 in (729.4 mm)	+0.14 in (+3.5 mm)	+0.48%
Sidewall height	3.79 in (96.3 mm)	3.86 in (98.0 mm)	+0.07 in (+1.8 mm)	+1.82%
Section width	10.83 in (275.0 mm)	9.65 in (245.0 mm)	−1.18 in (−30.0 mm)	−10.91%
Circumference	89.78 in (2,280.5 mm)	90.22 in (2,291.5 mm)	+0.43 in (+11.0 mm)	+0.48%
Revolutions per mile	706	702	−3	−0.48%

True speed with the speedometer calibrated for 275/35R21
Indicated	Actual on 275/35R21	Actual on 245/40R21	Difference
30 mph	30.0 mph	30.1 mph	+0.1 mph
60 mph	60.0 mph	60.3 mph	+0.3 mph
75 mph	75.0 mph	75.4 mph	+0.4 mph

Ride-height change: 245/40R21 raises the vehicle by 0.07 in (1.8 mm) at the axle (half the +3.5 mm diameter difference).

245/40R21 is 0.14 in taller than 275/35R21 and 30 mm narrower, a 0.48% difference in overall diameter.

With the speedometer calibrated for 275/35R21, an indicated 60 mph is actually 60.3 mph on 245/40R21, and revolutions per mile change from 706 to 702.

Both sizes mount on 21-inch wheels.

Size pages

Other comparisons with 275/35R21

Other comparisons with 245/40R21

Source: Calculated from standard tire geometry formulas. Verified 2026-09-27. How these numbers are calculated.