315/70R17 vs 35x12.50R17 — both sold as 35s, and one is not

Walk into a tire shop with "35s" as a requirement and you can be sold either of these. They are not the same tire, and the gap runs the opposite way to the one people usually check for.

The widths match; the heights do not

This is the reverse of the usual metric-versus-flotation problem. Normally the two notations agree roughly on height and diverge on width. Here:

• Section width: 12.40 in against 12.50 in — a difference of 0.10 in, under one per cent.
• Overall diameter: 34.36 in against 35.00 in — a difference of 0.64 in, 1.86%.

So the two casings are effectively the same width, and the metric size is two-thirds of an inch shorter. Anyone comparing them on the shop floor by eye would find them near-indistinguishable, and anyone comparing them by name would assume they were identical.

Why 315/70R17 gets called a 35

Because 34.36 rounds to 34.5, and 34.5 is close enough to 35 that the trade uses the label. The size exists as the metric route to a 35-inch-class tire on a 17-inch wheel, and it is genuinely the largest common metric size on that wheel — the ladder above it thins out very quickly.

The honest description is that it is a 34-and-a-half, and that the 0.64 in it gives away is real money in ride height terms: 0.32 in at the axle.

What the difference actually does

1.86% of speedometer error — a shade over one mile an hour at motorway pace, and nineteen miles in every thousand the odometer does not record. Revolutions per mile: 586.9 against 576.2, about eleven apart.

Those are small numbers. The reason the pairing matters is not the driving consequence but the purchasing one — a set of four is a substantial sum, and buyers routinely price the two against each other believing they are quoting the same specification.

The sidewall difference is where the character lives

8.68 in against 9.00 in. The flotation size is a derived 72-series; the metric size is a stated 70-series. Very close in proportion, and both extremely tall by any standard other than a truck tire's.

That similarity is worth stating because it is unusual. On most metric-versus-flotation pairings the two tires are different shapes as well as different sizes. These two are the same shape at slightly different scales, which is precisely why they are so easy to confuse.

A tenth of an inch is smaller than the wheel's own contribution

12.40 in against 12.50 in is the narrowest width gap between a metric and a flotation size anywhere in this catalogue. It is also smaller than what the wheel underneath does to the figure: change rim width by half an inch and a casing's measured section moves by something like 0.2 in, twice the gap on paper. Two mounted tires can therefore come off the tape in either order, and arguing about the 0.10 in is arguing about the wrong term.

Which rim widths each casing is approved for is a per-tire specification on the manufacturer's data sheet — it is not encoded in the size, and it is not held here. Your own wheel's width is stamped on the back of the spoke.

Availability runs the other way

35x12.50R17 is one of the most heavily stocked off-road sizes in North America and is offered in nearly every aggressive pattern made. 315/70R17 is a much shorter list. If the decision is being made on tread rather than on two-thirds of an inch — and it usually should be — that asymmetry decides it long before the geometry does.

What is above and below

Above: 315/70R17 against a real 37, which is 2.64 in and a different conversation entirely. Below, the same confusion happens one size down between a metric 285 and a flotation 33, where the height gap narrows and the width gap widens — the mirror image of this page.

What the arithmetic cannot arbitrate

Whether either size clears your vehicle, what rating the individual tire carries, and whether your wheels suit a 12.40 or a 12.50 section. Note also that the 0.64 in above is a difference between two specifications rather than between two products: manufacturing spread within a single flotation size regularly exceeds it, and a well-worn 35 will be shorter than a new 315/70R17. Anyone whose decision genuinely turns on two-thirds of an inch is asking a question that a size code cannot answer.