TireSizesFinder

315/70R17 vs 285/70R18

285/70R18 is 0.65 in (16.6 mm) shorter than 315/70R17, a −1.90% diameter difference. With the speedometer set for 315/70R17, an indicated 60 mph is 58.9 mph on 285/70R18.

Is this a safe swap?

Diameter difference −1.90%: within the ±3% guideline

By overall diameter alone, 285/70R18 falls inside the ±3% range commonly used as a guideline when replacing 315/70R17. This check uses diameter only. It does not assess load index, speed rating, wheel width or clearance.

To-scale drawing: 315/70R17 (34.36 in) vs 285/70R18 (33.71 in)315/70R1734.36 in × 12.40 in285/70R1833.71 in × 11.22 in
Side profile and tread width, drawn to the same scale.
315/70R17 vs 285/70R18 differences
Measurement315/70R17285/70R18Difference% diff
Overall diameter34.36 in (872.8 mm)33.71 in (856.2 mm)−0.65 in (−16.6 mm)−1.90%
Sidewall height8.68 in (220.5 mm)7.85 in (199.5 mm)−0.83 in (−21.0 mm)−9.52%
Section width12.40 in (315.0 mm)11.22 in (285.0 mm)−1.18 in (−30.0 mm)−9.52%
Circumference107.95 in (2,742.0 mm)105.90 in (2,689.8 mm)−2.05 in (−52.2 mm)−1.90%
Revolutions per mile587598+11+1.94%
True speed with the speedometer calibrated for 315/70R17
IndicatedActual on 315/70R17Actual on 285/70R18Difference
30 mph30.0 mph29.4 mph−0.6 mph
60 mph60.0 mph58.9 mph−1.1 mph
75 mph75.0 mph73.6 mph−1.4 mph

Ride-height change: 285/70R18 lowers the vehicle by 0.33 in (8.3 mm) at the axle (half the −16.6 mm diameter difference).

285/70R18 is 0.65 in shorter than 315/70R17 and 30 mm narrower, a 1.90% difference in overall diameter.

With the speedometer calibrated for 315/70R17, an indicated 60 mph is actually 58.9 mph on 285/70R18, and revolutions per mile change from 587 to 598.

285/70R18 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 285/70R18

Source: Calculated from standard tire geometry formulas. Verified . How these numbers are calculated.