315/70R18 vs 35x12.50R20

35x12.50R20 is 0.36 in (9.2 mm) shorter than 315/70R18, a −1.02% diameter difference. With the speedometer set for 315/70R18, an indicated 60 mph is 59.4 mph on 35x12.50R20.

Is this a safe swap?

Diameter difference −1.02%: within the ±3% guideline

By overall diameter alone, 35x12.50R20 falls inside the ±3% range commonly used as a guideline when replacing 315/70R18. 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.

315/70R18 vs 35x12.50R20 differences
Measurement	315/70R18	35x12.50R20	Difference	% diff
Overall diameter	35.36 in (898.2 mm)	35.00 in (889.0 mm)	−0.36 in (−9.2 mm)	−1.02%
Sidewall height	8.68 in (220.5 mm)	7.50 in (190.5 mm)	−1.18 in (−30.0 mm)	−13.61%
Section width	12.40 in (315.0 mm)	12.50 in (317.5 mm)	+0.10 in (+2.5 mm)	+0.79%
Circumference	111.09 in (2,821.8 mm)	109.96 in (2,792.9 mm)	−1.14 in (−28.9 mm)	−1.02%
Revolutions per mile	570	576	+6	+1.03%

True speed with the speedometer calibrated for 315/70R18
Indicated	Actual on 315/70R18	Actual on 35x12.50R20	Difference
30 mph	30.0 mph	29.7 mph	−0.3 mph
60 mph	60.0 mph	59.4 mph	−0.6 mph
75 mph	75.0 mph	74.2 mph	−0.8 mph

Ride-height change: 35x12.50R20 lowers the vehicle by 0.18 in (4.6 mm) at the axle (half the −9.2 mm diameter difference).

35x12.50R20 is 0.36 in shorter than 315/70R18 and 3 mm wider, a 1.02% difference in overall diameter.

With the speedometer calibrated for 315/70R18, an indicated 60 mph is actually 59.4 mph on 35x12.50R20, and revolutions per mile change from 570 to 576.

35x12.50R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 315/70R18.

Size pages

Other comparisons with 315/70R18

Other comparisons with 35x12.50R20

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