225/60R18 vs 315/35R20

315/35R20 is 0.05 in (1.3 mm) taller than 225/60R18, a +0.18% diameter difference. With the speedometer set for 225/60R18, an indicated 60 mph is 60.1 mph on 315/35R20.

Is this a safe swap?

Diameter difference +0.18%: under 1%

By overall diameter alone, 315/35R20 is a near match for 225/60R18; 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.

225/60R18 vs 315/35R20 differences
Measurement	225/60R18	315/35R20	Difference	% diff
Overall diameter	28.63 in (727.2 mm)	28.68 in (728.5 mm)	+0.05 in (+1.3 mm)	+0.18%
Sidewall height	5.31 in (135.0 mm)	4.34 in (110.3 mm)	−0.97 in (−24.7 mm)	−18.33%
Section width	8.86 in (225.0 mm)	12.40 in (315.0 mm)	+3.54 in (+90.0 mm)	+40.00%
Circumference	89.94 in (2,284.6 mm)	90.10 in (2,288.7 mm)	+0.16 in (+4.1 mm)	+0.18%
Revolutions per mile	704	703	−1	−0.18%

True speed with the speedometer calibrated for 225/60R18
Indicated	Actual on 225/60R18	Actual on 315/35R20	Difference
30 mph	30.0 mph	30.1 mph	+0.1 mph
60 mph	60.0 mph	60.1 mph	+0.1 mph
75 mph	75.0 mph	75.1 mph	+0.1 mph

Ride-height change: 315/35R20 raises the vehicle by 0.03 in (0.6 mm) at the axle (half the +1.3 mm diameter difference).

315/35R20 is 0.05 in taller than 225/60R18 and 90 mm wider, a 0.18% difference in overall diameter.

With the speedometer calibrated for 225/60R18, an indicated 60 mph is actually 60.1 mph on 315/35R20, and revolutions per mile change from 704 to 703.

315/35R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 225/60R18.

Size pages

Other comparisons with 225/60R18

Other comparisons with 315/35R20

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