245/50R18 vs 275/35R20

275/35R20 is 0.07 in (1.7 mm) shorter than 245/50R18, a −0.24% diameter difference. With the speedometer set for 245/50R18, an indicated 60 mph is 59.9 mph on 275/35R20.

Is this a safe swap?

Diameter difference −0.24%: under 1%

By overall diameter alone, 275/35R20 is a near match for 245/50R18; 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.

245/50R18 vs 275/35R20 differences
Measurement	245/50R18	275/35R20	Difference	% diff
Overall diameter	27.65 in (702.2 mm)	27.58 in (700.5 mm)	−0.07 in (−1.7 mm)	−0.24%
Sidewall height	4.82 in (122.5 mm)	3.79 in (96.3 mm)	−1.03 in (−26.2 mm)	−21.43%
Section width	9.65 in (245.0 mm)	10.83 in (275.0 mm)	+1.18 in (+30.0 mm)	+12.24%
Circumference	86.85 in (2,206.0 mm)	86.64 in (2,200.7 mm)	−0.21 in (−5.3 mm)	−0.24%
Revolutions per mile	730	731	+2	+0.24%

True speed with the speedometer calibrated for 245/50R18
Indicated	Actual on 245/50R18	Actual on 275/35R20	Difference
30 mph	30.0 mph	29.9 mph	−0.1 mph
60 mph	60.0 mph	59.9 mph	−0.1 mph
75 mph	75.0 mph	74.8 mph	−0.2 mph

Ride-height change: 275/35R20 lowers the vehicle by 0.03 in (0.9 mm) at the axle (half the −1.7 mm diameter difference).

275/35R20 is 0.07 in shorter than 245/50R18 and 30 mm wider, a 0.24% difference in overall diameter.

With the speedometer calibrated for 245/50R18, an indicated 60 mph is actually 59.9 mph on 275/35R20, and revolutions per mile change from 730 to 731.

275/35R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 245/50R18.

Size pages

Other comparisons with 245/50R18

Other comparisons with 275/35R20

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