Section width, tread width and contact patch: three widths

Ask how wide a tire is and there are at least three defensible answers. They differ by inches, they matter for different questions, and only one of them appears in the size code.

Section width: the widest point of the inflated casing

This is the size code's number. 315/70R17 states 315 mm — 12.40 in — and it is measured across the tire at its widest point, sidewall bulge included, with no load on it.

On most tires that widest point is somewhere around the middle of the sidewall, not at the tread. Raised lettering and protective ribs are excluded from the figure by definition; where a tire has substantial sidewall lettering, the standards define a separate overall width that includes it, and that is the larger number.

Section width is the right figure for one question above all others: what the tire will foul. An inner fender liner, an upper control arm and a suspension strut all sit at sidewall height.

Tread width: what is actually patterned

The distance across the tread blocks, shoulder to shoulder. It is always narrower than the section width, and by an amount that varies with the tire's profile and shoulder design rather than with its size class.

It is not a standardised published dimension in the way section width is. Manufacturers who publish it call it tread width or tread arc width, and it can differ between two tires with identical size codes from different makers.

The comparison worth internalising: 315/70R17 at 12.40 in of section width and 35x12.50R17 at 12.50 in are within a tenth of an inch of each other on paper, and can differ by considerably more than that across the tread depending on how each manufacturer shaped the shoulder.

The contact patch: neither of the above

What is on the ground at any instant is smaller than both, and its area barely depends on the tire at all.

Air pressure supports the vehicle. Ignoring the load the sidewall itself carries, the patch area is approximately the load on that corner divided by the inflation pressure. A corner carrying 1,200 lb at 35 psi produces roughly 34 square inches of contact — about the area of a five-by-seven-inch photograph.

Change the tire and that area stays roughly where it was. What changes is its shape: a wider tire produces a wider, shorter patch, a narrower one a narrower, longer patch. This is why the intuition that a wider tire puts "more rubber on the road" is only half right — it puts the same area of rubber down in a different rectangle, and the reasons wider tires behave differently have more to do with that shape, and with the construction that comes with it, than with the amount.

Deflate the tire and the area genuinely does grow, which is the entire mechanism of airing down for soft ground.

Overall width, and the ribs nobody counts

Between section width and the widest thing on the tire sits a third published figure. Overall width includes the raised elements the section width excludes: model names, sidewall styling, and on off-road tires the protective rib moulded around the shoulder to take rock contact before the casing does.

On an aggressive light-truck tire those features can add a meaningful fraction of an inch per side, and they are what actually meets an obstacle. Manufacturers publish overall width alongside section width where the difference is significant, and for clearance work it is the more honest number of the two.

Which width answers which question

Question	Width that answers it
Will it rub the fender liner at full lock?	Section width, on your rim
Will it clear the leaf spring or the frame rail?	Section width
How much tread is patterned across the tire?	Tread width, from the manufacturer's data
How much rubber is on the ground?	Contact patch area — load ÷ pressure
Which of two sizes is wider?	Section width, since it is the only one both publish

Where the numbers on this site sit

Every width figure computed here is section width, derived from the size code: millimetres divided by 25.4 for metric sizes, taken directly for flotation ones. It is the only width that both notations state and both standards define identically, which is what makes it comparable.

Two caveats travel with it, and they compound:

• The figure applies on the tire's nominal measuring rim. Rim width to tire width has the correction.
• It is measured unloaded. Under load the sidewall bulges outward slightly at the bottom, so the widest point on a standing vehicle is lower and marginally wider than the specification.

Measuring your own

With the tire mounted and inflated, measure across the widest point of the sidewall with the vehicle jacked up, not with the weight on it — the standing measurement includes deflection bulge and will not compare with any published number. For clearance work, the standing measurement is actually the more useful of the two, so take both and know which is which.

For a first pass on whether a candidate size is wider than what you have, the comparison tool reports the section width difference between any two codes to a hundredth of an inch.