315/70R17 vs 275/70R18

275/70R18 is 1.20 in (30.6 mm) shorter than 315/70R17, a −3.51% diameter difference. With the speedometer set for 315/70R17, an indicated 60 mph is 57.9 mph on 275/70R18.

Is this a safe swap?

Diameter difference −3.51%: outside the ±3% guideline

By overall diameter alone, 275/70R18 falls outside the ±3% range commonly used as a guideline when replacing 315/70R17; speedometer and ride-height changes are larger. 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.

315/70R17 vs 275/70R18 differences
Measurement	315/70R17	275/70R18	Difference	% diff
Overall diameter	34.36 in (872.8 mm)	33.16 in (842.2 mm)	−1.20 in (−30.6 mm)	−3.51%
Sidewall height	8.68 in (220.5 mm)	7.58 in (192.5 mm)	−1.10 in (−28.0 mm)	−12.70%
Section width	12.40 in (315.0 mm)	10.83 in (275.0 mm)	−1.57 in (−40.0 mm)	−12.70%
Circumference	107.95 in (2,742.0 mm)	104.17 in (2,645.8 mm)	−3.78 in (−96.1 mm)	−3.51%
Revolutions per mile	587	608	+21	+3.63%

True speed with the speedometer calibrated for 315/70R17
Indicated	Actual on 315/70R17	Actual on 275/70R18	Difference
30 mph	30.0 mph	28.9 mph	−1.1 mph
60 mph	60.0 mph	57.9 mph	−2.1 mph
75 mph	75.0 mph	72.4 mph	−2.6 mph

Ride-height change: 275/70R18 lowers the vehicle by 0.60 in (15.3 mm) at the axle (half the −30.6 mm diameter difference).

275/70R18 is 1.20 in shorter than 315/70R17 and 40 mm narrower, a 3.51% difference in overall diameter.

With the speedometer calibrated for 315/70R17, an indicated 60 mph is actually 57.9 mph on 275/70R18, and revolutions per mile change from 587 to 608.

275/70R18 mounts on a 18-inch wheel, a plus-1 fitment from the 17-inch wheel of 315/70R17.

Size pages

Other comparisons with 315/70R17

Other comparisons with 275/70R18

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