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Size comparisons

37x12.50R17 vs 315/70R17 — going down, and why the percentage changes

The tallest common flotation size measured against the tallest common metric one, from the larger tire's point of view.

37x12.50R1737.00 in tall12.50 in wide315/70R1734.36 in tall12.40 in wide-2.64 in · -7.13%
Drawn to scale from the size codes. Solid outline is 37x12.50R17; dashed outline is 315/70R17. The bars below each tire are section width at the same scale.

315/70R17 is 2.64 in shorter than 37x12.50R17 (7.13%) and 0.10 in narrower. A speedometer calibrated for 37x12.50R17 would over-read by 7.13%.

Measured against each other

  • Overall diameter

    37.00 in34.36 in-2.64 in (-7.13%)

  • Section width

    12.50 in / 318 mm12.40 in / 315 mm-0.10 in (-0.79%)

  • Sidewall height

    10.00 in8.68 in-1.32 in

  • Circumference

    116.2 in108.0 in-8.3 in

  • Revolutions per mile

    545.1586.9+41.8

  • Ride height at the axle

    ——-1.32 in

  • Speedometer reading

    ——-7.13% over-read

Calibrated for 37x12.50R17, running 315/70R17

Dial reads 30
27.9 mph
Dial reads 50
46.4 mph
Dial reads 70
65.0 mph
Odometer counts 100
92.9 mi covered

Change either size in the comparison tool, or see the full speed table in the speedometer error calculator.

Swapping between two tires produces two different percentages depending on which one you started from. This page is the downward direction, and the number is 7.13%.

Why the figure is not 7.68%

Because a percentage needs a denominator, and the denominator is whatever the vehicle was calibrated for.

  • Starting from 315/70R17, at 34.36 in, the 2.64 in gap is 7.68% of the original.
  • Starting from 37x12.50R17, at 37.00 in, the same 2.64 in is 7.13% of the original.

Both are correct. They answer different questions, and confusing them is one of the most common errors in tire arithmetic. The one that matters is always the percentage relative to the tire the speedometer was calibrated for.

The direction of the error

Downward, so the speedometer reads high — and by more than seven per cent, which is a larger over-read than almost anything else on this site produces. Revolutions per mile rises from 545.1 to 586.9, and that single count is what every instrument on the vehicle is ultimately reading.

Over-reading is the less familiar direction and the more consequential one for anybody relying on the odometer, because the vehicle records more distance than it has covered.

What is being given up

1.32 in of sidewall per corner, and the same figure again at the axle, since the wheel is not changing. Section width barely moves: 12.50 in to 12.40 in, a tenth of an inch.

So a vehicle moving from the flotation size to the metric one keeps its stance, its wheel and its clearance beside the tire, and loses two and a half inches of height.

Why anyone would

Weight and availability, usually. A 37x12.50R17 in an aggressive pattern is a heavy tire, and the metric size is generally lighter, cheaper and offered in more highway-biased compounds. For a vehicle that has been re-geared for 37s, dropping to the metric size also moves the effective final drive back toward where it started.

The name each one carries

One is called a 37 and is 37.00 in. The other is often called a 35 and is 34.36 in. Between the two labels there is an implied two-inch gap; between the two tires there is a two-and-a-half-inch one. Both descriptions round in the direction that flatters, which is worth allowing for whenever sizes are being compared by name rather than by figure.

The upward version

315/70R17 against 37x12.50R17 is the same pair from the other end, and it is the version most people arrive looking for.

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