275/40R17 vs 275/35R18

275/35R18 is 0.08 in (2.1 mm) shorter than 275/40R17, a −0.32% diameter difference. With the speedometer set for 275/40R17, an indicated 60 mph is 59.8 mph on 275/35R18.

Is this a safe swap?

Diameter difference −0.32%: under 1%

By overall diameter alone, 275/35R18 is a near match for 275/40R17; 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.

275/40R17 vs 275/35R18 differences
Measurement	275/40R17	275/35R18	Difference	% diff
Overall diameter	25.66 in (651.8 mm)	25.58 in (649.7 mm)	−0.08 in (−2.1 mm)	−0.32%
Sidewall height	4.33 in (110.0 mm)	3.79 in (96.3 mm)	−0.54 in (−13.7 mm)	−12.50%
Section width	10.83 in (275.0 mm)	10.83 in (275.0 mm)	0.00 in (0.0 mm)	0.00%
Circumference	80.62 in (2,047.7 mm)	80.36 in (2,041.1 mm)	−0.26 in (−6.6 mm)	−0.32%
Revolutions per mile	786	788	+3	+0.32%

True speed with the speedometer calibrated for 275/40R17
Indicated	Actual on 275/40R17	Actual on 275/35R18	Difference
30 mph	30.0 mph	29.9 mph	−0.1 mph
60 mph	60.0 mph	59.8 mph	−0.2 mph
75 mph	75.0 mph	74.8 mph	−0.2 mph

Ride-height change: 275/35R18 lowers the vehicle by 0.04 in (1.0 mm) at the axle (half the −2.1 mm diameter difference).

275/35R18 is 0.08 in shorter than 275/40R17 and the same width, a 0.32% difference in overall diameter.

With the speedometer calibrated for 275/40R17, an indicated 60 mph is actually 59.8 mph on 275/35R18, and revolutions per mile change from 786 to 788.

275/35R18 mounts on a 18-inch wheel, a plus-1 fitment from the 17-inch wheel of 275/40R17.

Size pages

Other comparisons with 275/40R17

Other comparisons with 275/35R18

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