275/40R21 vs 245/45R21

245/45R21 is 0.02 in (0.5 mm) taller than 275/40R21, a +0.07% diameter difference. With the speedometer set for 275/40R21, an indicated 60 mph is 60.0 mph on 245/45R21.

Is this a safe swap?

Diameter difference +0.07%: under 1%

By overall diameter alone, 245/45R21 is a near match for 275/40R21; 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.

275/40R21 vs 245/45R21 differences
Measurement	275/40R21	245/45R21	Difference	% diff
Overall diameter	29.66 in (753.4 mm)	29.68 in (753.9 mm)	+0.02 in (+0.5 mm)	+0.07%
Sidewall height	4.33 in (110.0 mm)	4.34 in (110.3 mm)	+0.01 in (+0.3 mm)	+0.23%
Section width	10.83 in (275.0 mm)	9.65 in (245.0 mm)	−1.18 in (−30.0 mm)	−10.91%
Circumference	93.18 in (2,366.9 mm)	93.25 in (2,368.4 mm)	+0.06 in (+1.6 mm)	+0.07%
Revolutions per mile	680	679	0	−0.07%

True speed with the speedometer calibrated for 275/40R21
Indicated	Actual on 275/40R21	Actual on 245/45R21
30 mph	30.0 mph	30.0 mph
60 mph	60.0 mph	60.0 mph
75 mph	75.0 mph	75.0 mph

Ride-height change: 245/45R21 raises the vehicle by 0.01 in (0.3 mm) at the axle (half the +0.5 mm diameter difference).

245/45R21 is 0.02 in taller than 275/40R21 and 30 mm narrower, a 0.07% difference in overall diameter.

With the speedometer calibrated for 275/40R21, an indicated 60 mph is actually 60.0 mph on 245/45R21, and revolutions per mile change from 680 to 679.

Both sizes mount on 21-inch wheels.

Size pages

Other comparisons with 275/40R21

Other comparisons with 245/45R21

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