TireSizesFinder

325/30R21 vs 245/40R21

245/40R21 is 0.04 in (1.0 mm) taller than 325/30R21, a +0.14% diameter difference. With the speedometer set for 325/30R21, an indicated 60 mph is 60.1 mph on 245/40R21.

Is this a safe swap?

Diameter difference +0.14%: under 1%

By overall diameter alone, 245/40R21 is a near match for 325/30R21; the speedometer changes by less than 1%. This check uses diameter only. It does not assess load index, speed rating, wheel width or clearance.

To-scale drawing: 325/30R21 (28.68 in) vs 245/40R21 (28.72 in)325/30R2128.68 in × 12.80 in245/40R2128.72 in × 9.65 in
Side profile and tread width, drawn to the same scale.
325/30R21 vs 245/40R21 differences
Measurement325/30R21245/40R21Difference% diff
Overall diameter28.68 in (728.4 mm)28.72 in (729.4 mm)+0.04 in (+1.0 mm)+0.14%
Sidewall height3.84 in (97.5 mm)3.86 in (98.0 mm)+0.02 in (+0.5 mm)+0.51%
Section width12.80 in (325.0 mm)9.65 in (245.0 mm)−3.15 in (−80.0 mm)−24.62%
Circumference90.09 in (2,288.3 mm)90.22 in (2,291.5 mm)+0.12 in (+3.1 mm)+0.14%
Revolutions per mile703702−1−0.14%
True speed with the speedometer calibrated for 325/30R21
IndicatedActual on 325/30R21Actual on 245/40R21Difference
30 mph30.0 mph30.0 mph0.0 mph
60 mph60.0 mph60.1 mph+0.1 mph
75 mph75.0 mph75.1 mph+0.1 mph

Ride-height change: 245/40R21 raises the vehicle by 0.02 in (0.5 mm) at the axle (half the +1.0 mm diameter difference).

245/40R21 is 0.04 in taller than 325/30R21 and 80 mm narrower, a 0.14% difference in overall diameter.

With the speedometer calibrated for 325/30R21, an indicated 60 mph is actually 60.1 mph on 245/40R21, and revolutions per mile change from 703 to 702.

Both sizes mount on 21-inch wheels.

Size pages

Other comparisons with 325/30R21

Other comparisons with 245/40R21

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