305/65R17 vs 275/65R18

275/65R18 is 0.54 in (13.6 mm) shorter than 305/65R17, a −1.64% diameter difference. With the speedometer set for 305/65R17, an indicated 60 mph is 59.0 mph on 275/65R18.

Is this a safe swap?

Diameter difference −1.64%: within the ±3% guideline

By overall diameter alone, 275/65R18 falls inside the ±3% range commonly used as a guideline when replacing 305/65R17. 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.

305/65R17 vs 275/65R18 differences
Measurement	305/65R17	275/65R18	Difference	% diff
Overall diameter	32.61 in (828.3 mm)	32.07 in (814.7 mm)	−0.54 in (−13.6 mm)	−1.64%
Sidewall height	7.81 in (198.3 mm)	7.04 in (178.8 mm)	−0.77 in (−19.5 mm)	−9.84%
Section width	12.01 in (305.0 mm)	10.83 in (275.0 mm)	−1.18 in (−30.0 mm)	−9.84%
Circumference	102.45 in (2,602.2 mm)	100.77 in (2,559.5 mm)	−1.68 in (−42.7 mm)	−1.64%
Revolutions per mile	618	629	+10	+1.67%

True speed with the speedometer calibrated for 305/65R17
Indicated	Actual on 305/65R17	Actual on 275/65R18	Difference
30 mph	30.0 mph	29.5 mph	−0.5 mph
60 mph	60.0 mph	59.0 mph	−1.0 mph
75 mph	75.0 mph	73.8 mph	−1.2 mph

Ride-height change: 275/65R18 lowers the vehicle by 0.27 in (6.8 mm) at the axle (half the −13.6 mm diameter difference).

275/65R18 is 0.54 in shorter than 305/65R17 and 30 mm narrower, a 1.64% difference in overall diameter.

With the speedometer calibrated for 305/65R17, an indicated 60 mph is actually 59.0 mph on 275/65R18, and revolutions per mile change from 618 to 629.

275/65R18 mounts on a 18-inch wheel, a plus-1 fitment from the 17-inch wheel of 305/65R17.

Size pages

Other comparisons with 305/65R17

Other comparisons with 275/65R18

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