Axle ratios: tooth counts, tire diameter and restoring rpm

An axle ratio is not a setting. It is the physical count of teeth on the ring gear divided by the count on the pinion that drives it. A 41-tooth ring on an 11-tooth pinion gives 3.7272…, printed on the box as 3.73:1, and it means the driveshaft turns 3.73 times for one turn of the wheels.

Because it is a tooth count, the available ratios are not continuous. They come in the steps the gear makers cut for a given axle housing, which is why an arithmetically ideal answer of 4.07 has to be resolved to something you can buy.

What the ratio is doing in the driveline

Two things at once, and they are the same thing seen from opposite ends.

• It multiplies torque. Whatever reaches the pinion is multiplied by the ratio before it reaches the axle shafts.
• It divides speed. The wheels turn slower than the driveshaft by the same factor.

Numerically higher ratios — 4.56, 4.88 — multiply more and turn the engine faster at a given road speed. Numerically lower ones — 3.07, 3.21 — do the opposite. The everyday vocabulary is backwards from the numbers, which is why "higher gears" is ambiguous and "numerically higher" is worth the extra word.

Tire diameter is part of the final drive

The gear set converts driveshaft turns into wheel turns. The tire converts wheel turns into distance. Nothing in the driveline knows or cares which of those two you changed.

That is the whole reason this page sits on a tire arithmetic site. Fitting a taller tire lowers your effective final drive exactly as if you had installed a numerically lower ring and pinion, in the same proportion, with the same effect on engine speed, torque at the contact patch and the speed at which the transmission wants to shift.

Take 275/65R18 at 32.07 in and step to 35x12.50R18 at 35.00 in. That is +2.93 in, or 9.12% taller. Every revolution now covers 9.12% more ground, so at any road speed the engine turns 9.12% slower — and the torque reaching the road drops by the same fraction, because the tire is a longer lever against the axle shaft.

The restoring ratio, and the ratio you can actually buy

The arithmetic that undoes a diameter change is a single multiplication:

new ratio = old ratio × (new diameter ÷ old diameter)

Scaling by how many times each tire turns in a mile gives the identical answer, and that is the form the calculator here uses — so a gearing result can never contradict a size page.

Run it on the step above:

Factory ratio	Ratio that restores engine speed	Nearest ratio sold as a set
3.42:1	3.73:1	3.73:1
3.73:1	4.07:1	4.10:1
4.10:1	4.47:1	4.56:1

The first row is a coincidence worth pointing at: on a 9.12% increase, a 3.42 axle lands precisely on 3.73, an off-the-shelf ratio for most domestic housings. The other two rows land between catalogue steps, and there the choice is between slightly over-correcting and slightly under-.

In engine speed, which is what you actually feel

Same truck, a 3.73 axle, and a top gear of 0.70:1 for illustration.

• On 275/65R18 at 70 mph: 1,915 rpm.
• On 35x12.50R18 at 70 mph: 1,755 rpm.
• On 35s with 4.10 gears at 70 mph: 1,929 rpm — fourteen revolutions above where the truck started, from a ratio change that was never going to be exact.

Whether 160 rpm matters is a question about the engine's torque curve, the load being carried and the terrain, not about the tires. What the arithmetic settles is the size of the change, and that it is real rather than perceptual.

The catalogue is coarse, and the housing has opinions

Consecutive ratios in a typical range — 3.55, 3.73, 3.92, 4.10, 4.27, 4.56 — sit roughly 5 to 8% apart. There is no 4.07. So a restoring figure almost always resolves upward or downward, and the size of that residual error is usually a percent or two of engine speed either way.

Two mechanical constraints narrow the choice further. Most axle housings have a carrier break: below a certain ratio the ring gear bolts to one differential carrier casting, above it to a different one, because the pinion has shrunk and the ring gear has to sit closer to the axle centreline. Crossing that break turns a gear-set purchase into a carrier purchase as well. And at the numerically high end the pinion runs out of teeth — a 5.38 set has a very small pinion meshing with a very large ring, which is why the deepest ratios exist only for the larger housings.

None of that is arithmetic, and none of it is visible in a calculator's answer. It is the reason the restoring figure is a target rather than an order form.

Where the ratio is written down

Most manufacturers encode it on a tag bolted to the differential cover or on the axle housing itself, and many put an option code on the vehicle's build sticker. Some record it as a tooth count — "41/11" — rather than a decimal.

This site records factory axle ratio options on the platform pages where they could be sourced, and omits the field entirely where they could not. A ratio invented to fill a table produces wrong engine-speed figures on a page that looks authoritative, so the absence is deliberate.

Two things the ratio does not change

• Wheel speed relative to the ground. That is tire circumference alone. Whether a regear moves the speedometer depends on where the vehicle takes its speed signal: from the wheels, and the axle ratio is invisible to it; from the transmission output shaft, upstream of the ring and pinion, and the ratio is part of the calculation. Speedometer calibration works through both cases.
• Anything about whether a given tire fits. Clearance, load rating against the door placard and local road rules are three separate questions with three separate answers.

Working it out for your own case

The gear ratio calculator takes an original size, a new size and a factory ratio and returns the restoring ratio and its nearest catalogue neighbour. Transmission gear ratios covers what happens upstream of the axle, where a wide-ratio eight-speed changes the conversation entirely. Revolutions per mile explains the quantity all of this is actually computed from.