205/65R16 vs 275/40R18

275/40R18 is 0.17 in (4.3 mm) taller than 205/65R16, a +0.64% diameter difference. With the speedometer set for 205/65R16, an indicated 60 mph is 60.4 mph on 275/40R18.

Is this a safe swap?

Diameter difference +0.64%: under 1%

By overall diameter alone, 275/40R18 is a near match for 205/65R16; 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.

205/65R16 vs 275/40R18 differences
Measurement	205/65R16	275/40R18	Difference	% diff
Overall diameter	26.49 in (672.9 mm)	26.66 in (677.2 mm)	+0.17 in (+4.3 mm)	+0.64%
Sidewall height	5.25 in (133.3 mm)	4.33 in (110.0 mm)	−0.92 in (−23.2 mm)	−17.45%
Section width	8.07 in (205.0 mm)	10.83 in (275.0 mm)	+2.76 in (+70.0 mm)	+34.15%
Circumference	83.23 in (2,114.0 mm)	83.76 in (2,127.5 mm)	+0.53 in (+13.5 mm)	+0.64%
Revolutions per mile	761	756	−5	−0.63%

True speed with the speedometer calibrated for 205/65R16
Indicated	Actual on 205/65R16	Actual on 275/40R18	Difference
30 mph	30.0 mph	30.2 mph	+0.2 mph
60 mph	60.0 mph	60.4 mph	+0.4 mph
75 mph	75.0 mph	75.5 mph	+0.5 mph

Ride-height change: 275/40R18 raises the vehicle by 0.08 in (2.2 mm) at the axle (half the +4.3 mm diameter difference).

275/40R18 is 0.17 in taller than 205/65R16 and 70 mm wider, a 0.64% difference in overall diameter.

With the speedometer calibrated for 205/65R16, an indicated 60 mph is actually 60.4 mph on 275/40R18, and revolutions per mile change from 761 to 756.

275/40R18 mounts on a 18-inch wheel, a plus-2 fitment from the 16-inch wheel of 205/65R16.

Size pages

Other comparisons with 205/65R16

Other comparisons with 275/40R18

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