Axle ratio by VIN: the one vehicle class where axles are required

Federal regulation lays out, for each class of vehicle, the attributes that a manufacturer must make readable from the middle of the VIN. Run down the rows and the word axle appears precisely once, in the entry covering trailers, which must convey trailer type, body type, length and axle configuration.

Passenger cars are required to convey line, series, body type, engine type and restraint system. Trucks add chassis, cab type, brake system and weight rating. Multipurpose passenger vehicles, buses and motorcycles each have their own row. Not one of them asks for anything about the axles.

So the pickup whose ratio you are trying to establish belongs to the one broad category for which the regulation is silent on the subject. Nothing has gone wrong with your search.

These two get conflated constantly, and the confusion is understandable, since both are called axle something and both sound technical.

Configuration is architecture. It answers how many axles the vehicle has and which of them deliver power — a description that matters enormously for a commercial trailer and hardly at all for a family sedan. It is expressed in words or a shorthand notation, never as a decimal.

Ratio is a measurement. It states how many times the driveshaft rotates for one rotation of the wheels, and it is written as a figure like 3.08, 3.55 or 4.10. Knowing a truck has two axles tells you nothing whatsoever about which of those numbers applies to it.

A great many people go looking for a ratio, find the certification label on the driver's door, see a figure labeled with the word axle, and stop there. It is the wrong number, and the mistake is worth naming precisely.

That label states gross axle weight ratings — the maximum load each axle is permitted to carry, quoted in kilograms and pounds. The regulation governing it requires the rating for each axle in order from front to rear, and it requires the figure to be stated together with the tire size designation it was certified against.

A rating in the thousands is a capacity. A ratio is a small decimal, usually somewhere between about 2.7 and 4.9 on road vehicles. If the number you are looking at has a pound sign anywhere near it, you have found something else.

Inside the differential, a small pinion gear on the end of the driveshaft turns a much larger ring gear attached to the wheels. Divide the teeth on the ring by the teeth on the pinion and you have the ratio. A 3.73 means the driveshaft turns 3.73 times for every single turn of the wheels.

A numerically higher ratio multiplies engine torque more before it reaches the road, which improves acceleration and pulling ability while raising engine speed at any given road speed. A lower one does the reverse, favoring quiet, economical cruising. Neither is better in the abstract; they suit different jobs.

What makes this worth pinning down on a used vehicle is that the choice was often invisible at the point of sale and remains invisible now. Two trucks that look identical in a listing, with the same engine and the same cab, can be carrying different ratios.

The most direct documentary answer is bolted to the axle housing. Manufacturers commonly fit a metal tag under one of the differential cover bolts, and that tag is specific to the assembly rather than to the vehicle.

• A tag under a differential cover bolt, often stamped with the ratio directly
• A ring and pinion tooth count, which divides out to the ratio
• A build or assembly date, useful for spotting a replaced unit
• A stamped code on the housing tube itself on some designs
• A limited-slip indicator, where the axle was built with one
• A cast part number, which a parts counter can translate

The tag is also the first casualty of neglect. It corrodes, it gets painted over during undercoating, and it is routinely thrown away when a differential cover is replaced after a fluid change. Its absence is not evidence of anything except a car that has been serviced.

Manufacturers who fitted more than one ratio to a model line generally recorded the choice as a short code on an option or sales-code label. General Motors lists it among the codes on its parts identification label; Ford and the Stellantis brands each have their own conventions and their own label positions.

The catch is that these codes are not self-explanatory. A three-character entry means nothing without the reference table published for that make and model year, and the same characters can mean different things across brands and across generations. Read the code, then translate it against a table you trust for that specific vehicle rather than a general list found online.

This is the method that ends every argument, because it measures the parts rather than consulting a record about them. It needs a jack, stands, a piece of chalk and about ten minutes.

1. 1Raise both driven wheels clear of the groundSupport the vehicle properly on stands with the transmission in neutral and the parking brake released. Both wheels must be free to turn.
2. 2Mark the driveshaft and one tireA chalk line across the driveshaft and a corresponding mark on the tire sidewall and the ground beneath it.
3. 3Turn both wheels through exactly one revolutionRotate them together, in the same direction, until the tire mark returns to its starting position.
4. 4Count the driveshaft revolutionsThe number of times your chalk line comes around is the ratio. About three and three quarters turns indicates a 3.73; a shade over three means a 3.08.
5. 5Match to the nearest catalog ratioManufacturers offered a defined set of ratios per model, so an imprecise count still identifies which of them you have.

While you have the wheels in the air, the same setup reveals what kind of differential the vehicle has, which is often as useful as the ratio itself.

Turn one wheel by hand and watch the other. On an open differential the opposite wheel rotates the other way. On a limited-slip unit it tends to turn in the same direction, because clutches or gears inside are transmitting torque across. On a locked axle both are effectively tied together.

This matters at resale. A limited-slip differential was frequently a paid option, it changes how the vehicle behaves in poor conditions, and its clutches wear. A seller claiming one is present has just given you something you can verify in thirty seconds.

Differentials get swapped. Some are regeared deliberately to suit larger tires or heavier towing; others are replaced wholesale with a used assembly after a failure, and whatever ratio that donor carried came along with it.

Either way the factory record stops being accurate, and this is the strongest argument for measuring rather than looking up. A build record describes the day the vehicle was assembled and is silent about everything since.

A regeared vehicle that has not had its speedometer recalibrated will also read incorrectly, which quietly distorts the odometer reading you are being asked to pay for. That is worth checking on any truck sitting on obviously larger than standard tires.

1. 1Decode the VIN for the surrounding factsModel year, engine, body and drive type — the context you need before an axle code or a tooth count means anything.
2. 2Look for the differential tagUnder a cover bolt on the housing. Where it survives, this is the fastest documentary answer available.
3. 3Read the option label and translate the codeUsing a reference table specific to that make and model year rather than a general list.
4. 4Count it if anything disagreesTen minutes with a jack settles it against the hardware, which is the only source a swapped axle cannot mislead.
5. 5Check open recalls while you have the VINFree and federal, and worth doing on any used vehicle you are inspecting closely enough to jack up.