Revolutions per mile does not move in step with diameter

There are 63,360 inches in a mile. Divide that by a tire's circumference in inches and you have its revolutions per mile — the number of times the wheel turns to cover the distance.

235/85R16 has a circumference of 99.7 in and turns 635.6 times per mile. 35x12.50R17 has 110.0 in of circumference and turns 576.2 times. That pair of numbers, not the diameters, is what the rest of the arithmetic on this site is built from.

Where the figure comes from

Circumference is π times overall diameter, so the whole chain from a size code is short:

• sidewall height from width and aspect ratio,
• overall diameter from the sidewall (twice) and the wheel,
• circumference from diameter,
• revolutions per mile from 63,360 ÷ circumference.

π appears in the third step and cancels out of every comparison, which is why a diameter ratio and a circumference ratio always give the same answer.

The asymmetry nobody expects

Revolutions per mile is inversely proportional to diameter. Going from the 31.73 in 235/85R16 to the 35.00 in 35x12.50R17 is a 10.31% increase in height — but revolutions per mile falls from 635.6 to 576.2, a drop of a little over 9%.

The reason is arithmetic rather than physical: a 10.31% increase in diameter is a division by 1.1031, and 1 ÷ 1.1031 is 0.9065 — so the revolutions figure falls 9.35%, not 10.31%. The gap widens as the change gets bigger. On a 25% diameter increase the revolutions figure falls 20%.

This matters whenever the two get mixed in one calculation. A percentage taken from a diameter comparison cannot be applied directly to a revolutions figure and vice versa, and swapping them is the most common way a gearing spreadsheet ends up a percent or two out.

What actually counts the revolutions

On anything built since anti-lock brakes became standard, each wheel carries a toothed ring and a sensor that produces a fixed number of pulses per wheel revolution. The vehicle's modules divide that pulse rate by a stored constant to obtain road speed and accumulate it for the odometer.

The stored constant is the vehicle's assumption about revolutions per mile — fixed at the factory against the tire size on the door placard. It has no way to detect a different tire. Fit one and every consumer of that signal inherits the same proportional error: speedometer, odometer, trip computer, the shift schedule, cruise control, the traction and stability systems' notion of reference speed, and on many vehicles the tire-pressure monitor's rotation counting.

Published figures and computed figures

Tire manufacturers print a revolutions-per-mile figure in their data sheets, and it is generally a few percent higher than the geometric one for the same size, because theirs is measured on a loaded, rolling tire rather than calculated from the mould. Neither is wrong; they answer slightly different questions. Overall diameter and rolling radius sets out why the two differ.

Everything on this site is computed from geometry, identically for every size, so that any two sizes can be compared without one of them borrowing an advantage from a different measuring method.

Counting the whole service life

The figures are small enough to feel abstract until you accumulate them. At 635.6 revolutions per mile, a tire that reaches 50,000 miles has turned about 31.8 million times, each rotation flexing the sidewall through the contact patch once. The taller tire in the comparison above does the same distance in about 28.8 million rotations — roughly three million fewer flex cycles for the same journey.

That is not a durability claim, and tread compound and load dominate how long a tire lasts. It is simply what the number means when it is integrated over a realistic distance.

The metric equivalent

Outside the United States the same quantity is expressed as revolutions per kilometre: 1,000,000 divided by the circumference in millimetres. It is the same measurement in different units, and European tire data sheets quote it as standard. A size at 576.2 revolutions per mile is turning about 358 times per kilometre.

Where the figure earns its keep

• Gearing. Engine speed at a given road speed is revolutions per mile × transmission ratio × axle ratio × mph ÷ 60. The familiar shop shortcut with 336 in it is the same equation with 63,360 ÷ (60π) folded into a constant. Axle ratio basics works through the consequences.
• Odometer error. The ratio of two revolutions figures is exactly the ratio by which recorded distance drifts.
• Matching tires across an axle. Two tires of the same nominal size but different wear turn at slightly different rates, and on a limited-slip or full-time all-wheel-drive system that difference is absorbed by hardware rather than ignored.
• Spare tires. A compact spare turns dramatically faster than the tire it replaces, which is the real reason for its speed and distance markings. Spare tire sizes has the figures.

Every size page on this site states its revolutions-per-mile figure, and the comparison tool reports the difference between any two sizes directly.