How wide is a 315 tire?
Answer
12.40 in wide
The first number in 315/70R17 is section width in millimetres: 315 mm, which is 12.40 in. That is the widest point of the inflated casing, not the tread.
The working
- Section width
- 315 mm / 12.40 in
- Overall diameter
- 34.36 in
- Sidewall height
- 8.68 in
Metric finally catches the imperial standard
Flotation truck tires have been 12.50 in wide for as long as the notation has existed: the 33x12.50, the 35x12.50, the 37x12.50 all share it. Metric widths climb in 10 mm steps and 315 mm is the first that gets there — 12.40 in, one tenth short. Nothing narrower comes close: a 305 is 12.01 in and a 295 is 11.61 in.
What that tenth of an inch is worth
Almost nothing, on its own. Two tires 0.10 in apart across the shoulder present the same problem to the same wheel arch. What differs is everything else about the two casings — 315/70R17 is 34.36 in tall while a 35x12.50R17 is 35.00 in — so the width match is the reason they get compared and the height mismatch is the reason the comparison needs care.
Each 10 mm step is 0.39 inches
Going 305 to 315 adds 0.39 in of section. It also adds 0.55 in of diameter, because a 70-series sidewall converts part of any extra width into extra height and then counts it twice. In metric sizing, width and height are not independent choices — asking for a wider tire at the same aspect ratio always means asking for a taller one.
The mounted width depends on the rim as well
The 12.40 in figure is quoted at a nominal measuring rim. Fit the same tire on a wheel half an inch wider and the casing gains roughly two-tenths of an inch of section while giving up a little height. On a tire this size, wheel choice can therefore move the measured width across the boundary that the 0.10 in gap above describes — which is worth remembering before treating either figure as fixed.
Where 12.40 inches actually goes
The section is measured at the casing's widest point, which sits above the tread and outboard of it. On a tire this wide the consequences show up at the inner edge under steering lock as readily as at the fender, and they are governed by the vehicle rather than the tire. Section width is what this page can tell you. Whether 12.40 in of it will pass a control arm is something only a tape measure on your own truck, at lock and at full compression, can settle. Set a comparison to see two widths drawn to the same scale.