How to Tell Gear Ratio From a VIN Number

The Short Answer

In most cases, you cannot read the gear ratio directly from the VIN. The Vehicle Identification Number is the 17-character code used on US-market vehicles since the 1981 model year. It identifies the manufacturer, vehicle type, model year, assembly plant and serial number, and some manufacturers also encode the engine or body style in it. The axle gear ratio is usually not one of those fields. The axle gear ratio is the number of driveshaft turns needed to turn the wheels once, written as a figure such as 3.73:1.

The VIN still matters, because it is the key to the vehicle's factory build record. Once the VIN tells you what the vehicle is, you can find the ratio in three ways: decode the build or option data tied to that VIN, read the factory labels or axle tags on the vehicle, or measure the ratio yourself. The build record only shows how the vehicle left the factory. If the axle or gears have been replaced, the only reliable way to know the current ratio is to inspect or measure the axle.

"Gear ratio" can also mean the individual gear ratios inside the transmission. Those ratios are fixed by the transmission model. To find them, identify the transmission from the build data or the transmission's own identification tag, then look up that model's specifications from the manufacturer. The rest of this guide covers the axle (differential) ratio, which is what most people mean by this question.

Before You Start

The full 17-character VIN, found on the driver's side of the dashboard where it is visible through the windshield, on the driver's door jamb label, and on the title and registration; The vehicle's make, model and model year; For the physical checks: a flashlight, a wire brush or rag to clean tags, chalk or paint marker, a jack rated for the vehicle, jack stands, and wheel chocks; The owner's manual or factory service information for lifting points and differential details

Step 1: Decode the VIN to Confirm Exactly What You Have

Run the VIN through a VIN decoder to confirm the model year, model, trim, engine and drivetrain. Decoders based on federal VIN data give the basic configuration but usually not the axle ratio. This step makes sure that any spec you look up later applies to your vehicle. Axle ratio options often differ by engine, transmission, drive type, towing package and model year, so applying information from a different configuration can give you the wrong answer.

Step 2: Get the Factory Build Data Tied to the VIN

The most direct way to turn a VIN into a gear ratio is the factory build sheet or option list, which records the equipment installed when the vehicle was built, usually including the axle ratio. Here are common ways to get it:

Ask a franchised dealer for that brand to pull the build or option information using the VIN. Service departments can often look this up. Check whether the manufacturer offers a build sheet or window sticker lookup by VIN for your brand and model year. Use a paid vehicle history or build data service that offers factory option details. Check that the report actually lists axle ratio before relying on it. Look for the original window sticker, if the vehicle still has one. It may list the axle ratio as an option or package item.

Availability varies by manufacturer and model year. Older vehicles and some brands may have little or no build data available by VIN.

Step 3: Read the Option Codes and Labels on the Vehicle

Many vehicles carry a factory label that lists option codes, including the axle ratio. Where the label is and how the codes are written depends on the manufacturer:

GM vehicles often have a Service Parts Identification label, usually in the glove box, on the inside of a door, or near the spare tire. It lists three-character Regular Production Option (RPO) codes. Axle ratio codes are typically among those beginning with G. Match the code against a GM RPO reference for your model year. Ford vehicles typically show an axle code on the safety certification label in the driver's door jamb. Decode it with Ford reference material for your model and year. Other manufacturers use their own option or sales codes. Check the door jamb and glove box labels, then decode them with that brand's service information or a dealer.

Code meanings differ between brands and can change between model years. Always decode a code with a reference that matches your vehicle's make, model and year.

Step 4: Check the Axle Itself for a Tag or Stamping

Many differentials have a metal tag held by one of the cover bolts or a stamped code on the axle housing or tube. Depending on the manufacturer, this tag may show the ratio directly or as a code. Park on level ground, set the parking brake, and let the axle cool if the vehicle has just been driven. Clean the area with a wire brush or rag and read the tag with a flashlight. You don't need to lift the vehicle if you can see the tag safely from the ground. Tags can be lost, painted over or swapped during repairs, so treat a tag as evidence rather than proof if the vehicle's history is unknown.

Step 5: Measure the Ratio by Turning the Driveshaft

If the paperwork, labels and tags are missing or you suspect the gears were changed, you can measure the ratio directly. The method below is for a rear-wheel-drive axle. The same idea applies to other axles, but the setup and lifting procedure may differ. Follow your vehicle's service information, especially on four-wheel-drive, all-wheel-drive and front-wheel-drive vehicles.

1) Park on firm, level ground and chock the front wheels on both sides. 2) Raise the rear of the vehicle at the manufacturer's specified lifting points and support it securely on jack stands. Never rely on the jack alone, and never get under a vehicle held only by a jack. Stop if the ground is soft or uneven or the vehicle shifts. 3) Put the transmission in neutral and release the parking brake so the rear wheels and driveshaft can turn. On a four-wheel-drive vehicle, follow the service information for putting the transfer case in a state that lets the rear driveshaft turn freely. The chocked front wheels must keep the vehicle from rolling. 4) Make a chalk mark on one rear tire at the top and a matching mark on the driveshaft. 5) Rotate both rear wheels together, in the same direction, through exactly one full turn, and count the driveshaft turns, including any partial turn. If only one wheel can turn and the axle has an open differential, turning that one wheel once and counting gives half the ratio, so double your count. 6) For better accuracy, turn the wheels 10 full times, count the driveshaft turns, and divide by 10. 7) Compare your result with the ratios offered for your vehicle's configuration. A measured value near 3.7, for example, points to a 3.73 ratio if that ratio was offered. 8) Lower the vehicle safely, and put the transmission back in park (or in gear for a manual) and reapply the parking brake before removing the chocks.

Hand counting is approximate. It can tell apart ratios that are far apart, but it may not reliably separate two ratios that are close to each other.

Step 6: Count Gear Teeth for a Definitive Answer

The exact ratio comes from dividing the number of teeth on the ring gear by the number of teeth on the pinion gear. Counting them means removing the differential cover or inspecting the gears during service, which drains the gear oil. The axle then has to be resealed and refilled with the fluid type and amount the manufacturer specifies. Some axles also need a friction modifier. Many owners have a qualified technician do this, often during a scheduled differential fluid change. If the gear set has been replaced, the ring gear may also be marked with its tooth counts or ratio.

How to Know You Have the Right Answer

You can be confident in the ratio when two independent sources agree, for example the build data from the VIN and the axle tag, or the door jamb or option label and a driveshaft count. If the factory records disagree with what you measure on the axle, trust the physical measurement for the vehicle as it is now. The axle may have been changed since it left the factory. For work where the ratio matters, such as ordering parts, recalibrating the speedometer or setting up towing, confirm the figure with service information or a dealer for your exact vehicle.