Is 315/70R17 a 35 inch tire?

Closer to the size below it than the size it is named after

34.36 in sits 0.64 in under 35.00 in. It also sits 0.55 in above 305/70R17, which nobody calls a 35. By distance alone, this tire has more claim to being a tall 34 than a short 35 — the label attached to it in catalogues is the nearest round number upward, not the nearest round number.

Width is why the label sticks

12.40 in of section against the 12.50 in of a 35x12.50R17: a tenth of an inch apart. Standing in a wheel well, the two look interchangeable, because the dimension your eye reads first is the one where they genuinely match. Height is the dimension that differs, and height is the one that is hardest to judge by looking.

What the 0.64 in changes

Against a speedometer calibrated for a true 35, the shorter tire reads 1.82% high — an indicated 70 mph is 68.7 mph. Ride height at the axle drops by about a third of an inch, and the shortfall is entirely sidewall: 8.68 in against the 9.00 in a 35x12.50R17 carries, so roughly a third of an inch goes missing above the wheel and the same again below it. Neither figure is dramatic, and both are consistently in the direction people do not expect: someone who fitted 315/70R17 believing they had 35s has a slightly lower truck and a slightly optimistic odometer, not the reverse.

Where it stands among the alternatives

Nothing metric on a 17-inch wheel goes higher. Ask for more than 34.36 in and the next thing available is described in inches rather than millimetres. That is the real reason the 35 label attached itself here: not because the tire measures 35 in, but because it is the last stop before the notation changes under you. Compare it against the genuine article and the whole difference is vertical.