Gear Ratio Calculator

What the tool is doing

Changing tire diameter changes how far the vehicle travels per turn of the driveshaft, which is arithmetically identical to changing the axle ratio. A tire 10% taller behaves like an axle ratio 10% numerically lower: the engine turns more slowly at a given road speed, and it has less mechanical advantage at every point.

Engine speed is computed from road speed, the tire's revolutions per mile, and the two ratios between the engine and the wheels; the restoring ratio is the axle ratio that puts engine speed back where it started once the tire has changed. Both formulas, and the constant hiding inside the shop version most people have seen, are written out at how every figure on this site is computed.

Two things about that are worth knowing while you read the result. The revolutions-per-mile figure feeding this tool comes from the same geometry function that produces the diameter on every size page, so a gearing number here can never disagree with a diameter number elsewhere on the site. And the restoring ratio is a continuous value — it is whatever the arithmetic says, not a ratio anyone sells — which is why the result also names the nearest ratio that is actually manufactured.

A worked example

Take 265/70R17 on a 3.73:1 axle and fit 35x12.50R17. The diameters are 31.61 in and 35.00 in, and revolutions per mile fall from 638.1 to 576.2 — the tire is 10.74% taller. The ratio that restores the original engine speed is 4.13:1, and the nearest ratio genuinely sold as a ring-and-pinion set is 4.10:1.

Read the seventh-gear row of the table at 70 mph on that 4.10 axle — the 0.84 ratio — and the engine turns 2,315 rpm. The eighth row, a 0.67 top gear, is the one that actually decides a motorway cruise, and it gives 1,847.

Go the smaller step to 33x12.50R17 from the same starting point and the restoring ratio is 3.89:1, with 3.91:1 the nearest set sold — which is why 33s are so often described as needing nothing and 35s as needing a re-gear.

The transmission ratios are yours to replace

The table opens on a generic eight-speed set — 4.71 down to 0.67 — attributed to no transmission at all. That is deliberate: manufacturer ratio sets vary by model year and by drivetrain option in ways that are not reliably published, and printing an unsourced set under a named gearbox would be the invented figure this site's accuracy rule forbids.

So the set is a field rather than a fixture. Transmission ratios, first to top takes a comma-separated list — anything from three to twelve gears, each between 0.1 and 12.0 — and the table, the gear labels and every engine-speed figure recompute from what you type. A four-speed and a ten-speed both work; the top gear is labelled as such whichever position it lands in. Ratios that are unreadable or out of range are dropped rather than silently corrected, and emptying the field brings the generic set back.

Like every other control on this site, it is carried in the URL, so a link to a result is a link to that result and not to the default. Your own ratios are usually printed in the owner's manual or stamped on the transmission tag.

What "restores the original rpm" means, and what it does not

It means exactly one thing: at the same road speed in the same gear, the engine turns at the same speed it did before. That is a statement about arithmetic.

It is not a recommendation. Whether re-gearing is worth doing depends on what you are driving, how you use it, what the transmission does with torque converter lock-up, and what the work costs. This tool reports the number and the nearest available ratio, and stops there.