Transmission ratios, overall ratio and crawl ratio

Between the crankshaft and the road there is a chain of ratios, and they multiply. Whatever the engine is doing arrives at the tire divided by their product, and whatever torque it makes arrives multiplied by the same figure.

The chain, in order: transmission gear, transfer case if the vehicle has one, final drive at the axle, then the tire converting rotation into distance.

Overall ratio is the number that matters

A transmission ratio on its own tells you very little. Multiply it by the axle ratio and you have the overall ratio for that gear, which is directly comparable across vehicles:

• A direct-drive gear of 1.00:1 behind a 3.55 axle gives an overall 3.55.
• A top overdrive of 0.65:1 behind the same axle gives 2.31.
• A first gear of 4.71:1 behind it gives 16.7.

Two vehicles with the same axle ratio and different gearbox spreads are not geared the same, and two with different axle ratios can be geared identically. The axle ratio alone is a poor way to compare, and it is the way everybody compares.

Engine speed, in one line

rpm = mph × revolutions-per-mile × transmission ratio × axle ratio ÷ 60

On 265/70R17, with a 3.55 axle:

• In a 1.00:1 direct gear at 70 mph: 2,643 rpm.
• In a 0.65:1 overdrive at 70 mph: 1,718 rpm.
• In a 2.44:1 intermediate gear at 30 mph: 2,764 rpm.

The tire enters through the revolutions-per-mile term and nowhere else, which is why a diameter change and a ratio change are the same kind of change as far as the engine is concerned.

A second way to hold the same relationship is to ask how far the vehicle travels for one turn of the crankshaft. In that direct gear on that axle it is 27.97 in — call it two and a third feet per engine revolution. Halve the gear ratio and it doubles; make the tire 10% taller and it grows 10%. Both levers act on the same number, which is the whole reason they are interchangeable.

Why gear count changed the argument

A four-speed automatic with a 0.70 top gear had one overdrive and no alternatives. If a tire change dropped cruise rpm below where the engine made useful torque, the only fix was at the axle.

A modern eight-, nine- or ten-speed has two or three overdriven ratios, often with a top gear numerically far taller — figures in the 0.6s and below are ordinary. The transmission can respond to a taller tire by declining to use its highest gear, and it will do exactly that, silently, on a gradient or with a trailer attached.

That has two consequences worth naming:

• The symptom of being under-geared changed. It used to be low cruise rpm. Now it is a transmission that hunts between the top two gears, or refuses the top one, or holds a lower gear on the same hill it used to take in top.
• The remedy is less often a regear. Where the gearbox has ratio steps close enough together, the vehicle absorbs a moderate tire increase by shifting differently.

Torque converters and the ratio that is not a gear

An automatic adds one more multiplier that no ratio chart lists. A torque converter multiplies torque whenever there is slip across it — most of all from a standstill, tapering to none once the lock-up clutch engages. Effective reduction at launch is therefore higher than first gear alone suggests, and unlike a gear, it is not a fixed number.

Once the converter locks, the chain is purely mechanical and the arithmetic above applies exactly. Everything below lock-up is an approximation, which is why cruise-rpm figures are quoted at highway speed in a locked top gear and launch behaviour is not quoted at all.

Crawl ratio: three multiplications, low range included

For a vehicle with a two-speed transfer case, the number that matters off pavement is the crawl ratio — the overall reduction in first gear, low range:

crawl ratio = first gear × transfer case low range × axle ratio

With a 4.71:1 first gear, a 2.72:1 low range and a 4.10:1 axle, that is about 52.5:1. Swap in a 3.60:1 first gear from a different transmission and it falls to about 40.1:1 — the same axle and transfer case, a third less reduction, from one gear in the chain.

Fitting taller tires reduces the effective crawl ratio in exact proportion to the diameter increase, because the last link in the chain got longer. A 10% taller tire turns a 52.5:1 crawl ratio into roughly 47.7:1.

Ratio spread, and what a wide gearbox is buying

Divide first gear by top gear and you have the spread. A four-speed automatic with a 2.84 first and a 0.70 top spans about 4.1:1. A ten-speed with a 4.70 first and a 0.63 top spans nearly 7.5:1.

The spread is what lets a manufacturer fit a numerically low axle ratio for economy without leaving the vehicle unable to pull away with a load. It also explains an apparent contradiction on modern trucks: axle ratios have crept lower over two decades while towing ratings have climbed, because the gearbox is doing work the axle used to do.

For anyone changing tire size, the spread is the margin available. A wide-ratio gearbox absorbs a diameter increase by using a different gear more often; a narrow one has nowhere to go and passes the whole change through to the engine.

Where the ratios are written down

Transmission ratio sets are published in service manuals and in manufacturers' technical literature, and they change between model years, engine choices and duty ratings more often than axle ratios do. Two vehicles with the same badge and the same axle can have entirely different gear sets.

That variability is why this site omits transmission ratios from its platform records unless they could be confirmed against a source. An invented ratio produces a wrong rpm figure on a page that looks authoritative, and rpm arithmetic is exactly what somebody would use such a page for. The field is absent on the vehicle pages where it could not be sourced, and it says so there.

If you know your own set, the gear ratio calculator takes a transmission ratio, an axle ratio and a tire size and returns engine speed at any road speed — which is more reliable than any published table, because it uses your numbers.