TireSizesFinder

245/45R18 vs 275/30R20

275/30R20 is 0.19 in (4.7 mm) shorter than 245/45R18, a −0.69% diameter difference. With the speedometer set for 245/45R18, an indicated 60 mph is 59.6 mph on 275/30R20.

Is this a safe swap?

Diameter difference −0.69%: under 1%

By overall diameter alone, 275/30R20 is a near match for 245/45R18; 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: 245/45R18 (26.68 in) vs 275/30R20 (26.50 in)245/45R1826.68 in × 9.65 in275/30R2026.50 in × 10.83 in
Side profile and tread width, drawn to the same scale.
245/45R18 vs 275/30R20 differences
Measurement245/45R18275/30R20Difference% diff
Overall diameter26.68 in (677.7 mm)26.50 in (673.0 mm)−0.19 in (−4.7 mm)−0.69%
Sidewall height4.34 in (110.3 mm)3.25 in (82.5 mm)−1.09 in (−27.7 mm)−25.17%
Section width9.65 in (245.0 mm)10.83 in (275.0 mm)+1.18 in (+30.0 mm)+12.24%
Circumference83.82 in (2,129.1 mm)83.24 in (2,114.3 mm)−0.58 in (−14.8 mm)−0.69%
Revolutions per mile756761+5+0.70%
True speed with the speedometer calibrated for 245/45R18
IndicatedActual on 245/45R18Actual on 275/30R20Difference
30 mph30.0 mph29.8 mph−0.2 mph
60 mph60.0 mph59.6 mph−0.4 mph
75 mph75.0 mph74.5 mph−0.5 mph

Ride-height change: 275/30R20 lowers the vehicle by 0.09 in (2.4 mm) at the axle (half the −4.7 mm diameter difference).

275/30R20 is 0.19 in shorter than 245/45R18 and 30 mm wider, a 0.69% difference in overall diameter.

With the speedometer calibrated for 245/45R18, an indicated 60 mph is actually 59.6 mph on 275/30R20, and revolutions per mile change from 756 to 761.

275/30R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 245/45R18.

Size pages

Other comparisons with 245/45R18

Other comparisons with 275/30R20

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