What is 35x12.50R17 in metric?

The famous answer is 0.64 in short

This conversion has a widely circulated answer — 315/70R17 — and it is worth knowing how approximate it is. A 35x12.50R17 stands 35.00 in. A 315/70R17 stands 34.36 in. The two are 0.10 in apart in section width, which is why they are so readily substituted, and 0.64 in apart in height, which is why the substitution is not free.

The exact conversion, and why it is unbuyable

12.50 in of section is 317.5 mm; a 9.00 in sidewall on that section is an aspect ratio of 72. Written in metric the tire is 318/72R17. No such size is produced. Metric truck tires are built around widths ending in 5 and aspect ratios in multiples of five, and this one falls between the rungs on both counts.

Nothing metric on a 17-inch wheel gets any closer

There is nothing above 315/70R17 in the metric range on this rim, so the list of alternatives only runs downhill from here:

• 305/70R17 — 33.81 in, 1.19 in short.
• 295/70R17 — 33.26 in, 1.74 in short.
• 255/80R17 — 33.06 in, 1.94 in short and much narrower.

If a genuine 35.00 in is required on a 17, the answer is a flotation size, because the metric catalogue simply stops before it.

The gap, counted rather than measured

35x12.50R17 turns 576.2 times to cover a mile; 315/70R17 turns 586.9. Just under eleven extra rotations per mile is the same 0.64 in stated as a count instead of a length, and it is the form the vehicle's electronics actually work in. All of it comes out of the sidewall — 9.00 in on the flotation size against 8.68 in on the metric one.

What the 0.64 in is worth

Against a speedometer set for a true 35, the 315 reads 1.82% high, and it sits a third of an inch lower at the axle. Put the pair side by side and the outlines nearly overlap across the shoulders while separating at the crown — the visual reason so many people believe the substitution is exact.