VINs: The Encoding Stamped Into Steel – Revved | The CarGurus Engineering Blog

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VINs: The Encoding Stamped Into Steel

Written by

Brendan McLoughlin

in

Automotive

Do I have a favorite standard? Silly question. How could I not? There are so many great ones, each with its own weirdly fascinating story.

In a previous job, I spent a lot of time getting to know ECMA-262 (the JavaScript language specification). Lately, though, I’ve been acquiring a taste for encoding standards. Encodings are fascinating. They’re all about constraints and information density. How much can you pack into a fixed space? What can you safely leave out? and what existing patterns do you need to work around?

Of course, encodings only work if everyone agrees to use them. Check out the later chapters of Jing Tsu’s excellent Kingdom of Characters for a reminder that even a clever, seemingly-dry technical standard like Unicode can turn into a messy game of politics the moment the whole world has to agree.

But in the automotive world, it’s hard to beat ISO 3779: the international standard for the humble Vehicle Identification Number (VIN). If anyone ever writes the tell-all book about how that standard came to be, I’ll be the first to preorder it.

Because a VIN is so much more than a serial number. It’s an encoding: a compact little artifact that carries pieces of a vehicle’s manufacturing story. Why would we want that? Let me tell you a story…

Alice and the bicycle shop

Imagine a small town with a local bicycle shop. The shop owner, Alice, makes custom bikes. Alice is proud of the quality of her work and decides to offer a 1 year warranty on her bikes. To accomplish this she needs to keep track of her bikes to know if they are still under warranty. At first, she only needs to keep track of a few bikes at a time, so she uses a simple numbering system: Bike 1, Bike 2, Bike 3, and so on.

As Alice’s business grows, she starts making different types of bikes: mountain bikes, road bikes, and city bikes. She also begins to source parts from various suppliers. She grows tired of looking up bike numbers from her record to understand what they are and when they were manufactured. So she creates a more sophisticated system to track not just the number of bikes, but also their types, components, and manufacturing dates.

Alice develops a new identification system. Each bike now gets a 10-character code:

2 letters for the bike type (MB for mountain bike, RB for road bike, CB for city bike)

4 numbers for the date of manufacture (MMYY)

4 numbers for the sequential production number

So, a mountain bike made in March 2023, being the 15th bike of that type, would be: MB032300015.

A month later, a customer rolls in with a bike whose basket has come loose. Alice reads the tag: CB062500128. She immediately knows it’s a city bike built in June 2025, still within warranty, and later she can pull the exact build sheet to see which basket it shipped with. Alice still needs her records, but the code gives her enough context to answer the warranty question without flipping through paperwork.

Alice isn’t alone in discovering this pattern. Serial numbers for high-value manufactured products are common across industries, from power tools to medical devices to industrial equipment. They make it easier to trace inventory, manage warranties and service history, fight counterfeits, support theft recovery, and do targeted recalls when something goes wrong.

From serial numbers to VINs

In the automotive world, serial numbers evolved into something more sophisticated: the VIN (vehicle identification number). In the United States, we follow ISO 3779’s structure, but U.S. regulations are more prescriptive about how some of those fields are used. And that’s where VINs get fun: they’re not just serial numbers, they’re a shared contract, something thousands of independent companies can read, type, validate, and exchange without needing a central database.

A VIN is a 17-character code, broken into sections:

Positions 1-3 (World Manufacturer Identifier): Identifies the manufacturer, and the first character is commonly used as a country/region-of-origin indicator. For example, vehicles with VINs starting with 1/4/5 are associated with the United States, while J is Japan.

Positions 4-8 (Vehicle Descriptor Section): Platform, model, body style, and often the engine type when multiple options exist.

Position 9: Check digit (more on this in a moment)

Position 10: Model year

Position 11: Assembly plant code

Positions 12-17: Sequential production number for that model at that plant in that year

Designed for the real world

The letters O (o), I (i), and Q (q) never appear in VINs. VINs get handwritten on insurance forms, read aloud over phone calls to DMVs, transcribed from photos of dashboard plates. By excluding characters that are easily confused with numerals (I/1, O/0, Q/9), the designers eliminated an entire class of transcription errors. The...