37x13.50R17 vs 315/70R18

315/70R18 is 1.64 in (41.6 mm) shorter than 37x13.50R17, a −4.43% diameter difference. With the speedometer set for 37x13.50R17, an indicated 60 mph is 57.3 mph on 315/70R18.

Is this a safe swap?

Diameter difference −4.43%: outside the ±3% guideline

By overall diameter alone, 315/70R18 falls outside the ±3% range commonly used as a guideline when replacing 37x13.50R17; 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.

37x13.50R17 vs 315/70R18 differences
Measurement	37x13.50R17	315/70R18	Difference	% diff
Overall diameter	37.00 in (939.8 mm)	35.36 in (898.2 mm)	−1.64 in (−41.6 mm)	−4.43%
Sidewall height	10.00 in (254.0 mm)	8.68 in (220.5 mm)	−1.32 in (−33.5 mm)	−13.19%
Section width	13.50 in (342.9 mm)	12.40 in (315.0 mm)	−1.10 in (−27.9 mm)	−8.14%
Circumference	116.24 in (2,952.5 mm)	111.09 in (2,821.8 mm)	−5.15 in (−130.7 mm)	−4.43%
Revolutions per mile	545	570	+25	+4.63%

True speed with the speedometer calibrated for 37x13.50R17
Indicated	Actual on 37x13.50R17	Actual on 315/70R18	Difference
30 mph	30.0 mph	28.7 mph	−1.3 mph
60 mph	60.0 mph	57.3 mph	−2.7 mph
75 mph	75.0 mph	71.7 mph	−3.3 mph

Ride-height change: 315/70R18 lowers the vehicle by 0.82 in (20.8 mm) at the axle (half the −41.6 mm diameter difference).

315/70R18 is 1.64 in shorter than 37x13.50R17 and 28 mm narrower, a 4.43% difference in overall diameter.

With the speedometer calibrated for 37x13.50R17, an indicated 60 mph is actually 57.3 mph on 315/70R18, and revolutions per mile change from 545 to 570.

315/70R18 mounts on a 18-inch wheel, a plus-1 fitment from the 17-inch wheel of 37x13.50R17.

Size pages

Other comparisons with 37x13.50R17

Other comparisons with 315/70R18

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