TireSizesFinder

315/75R17 vs 315/70R18

315/70R18 is 0.24 in (6.1 mm) shorter than 315/75R17, a −0.67% diameter difference. With the speedometer set for 315/75R17, an indicated 60 mph is 59.6 mph on 315/70R18.

Is this a safe swap?

Diameter difference −0.67%: under 1%

By overall diameter alone, 315/70R18 is a near match for 315/75R17; the speedometer changes by less than 1%. This check uses diameter only. It does not assess load index, speed rating, wheel width or clearance.

To-scale drawing: 315/75R17 (35.60 in) vs 315/70R18 (35.36 in)315/75R1735.60 in × 12.40 in315/70R1835.36 in × 12.40 in
Side profile and tread width, drawn to the same scale.
315/75R17 vs 315/70R18 differences
Measurement315/75R17315/70R18Difference% diff
Overall diameter35.60 in (904.3 mm)35.36 in (898.2 mm)−0.24 in (−6.1 mm)−0.67%
Sidewall height9.30 in (236.3 mm)8.68 in (220.5 mm)−0.62 in (−15.7 mm)−6.67%
Section width12.40 in (315.0 mm)12.40 in (315.0 mm)0.00 in (0.0 mm)0.00%
Circumference111.85 in (2,840.9 mm)111.09 in (2,821.8 mm)−0.75 in (−19.2 mm)−0.67%
Revolutions per mile566570+4+0.68%
True speed with the speedometer calibrated for 315/75R17
IndicatedActual on 315/75R17Actual on 315/70R18Difference
30 mph30.0 mph29.8 mph−0.2 mph
60 mph60.0 mph59.6 mph−0.4 mph
75 mph75.0 mph74.5 mph−0.5 mph

Ride-height change: 315/70R18 lowers the vehicle by 0.12 in (3.0 mm) at the axle (half the −6.1 mm diameter difference).

315/70R18 is 0.24 in shorter than 315/75R17 and the same width, a 0.67% difference in overall diameter.

With the speedometer calibrated for 315/75R17, an indicated 60 mph is actually 59.6 mph on 315/70R18, and revolutions per mile change from 566 to 570.

315/70R18 mounts on a 18-inch wheel, a plus-1 fitment from the 17-inch wheel of 315/75R17.

Size pages

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