Dear Tom and Ray:

The other day, I stopped my Toyota Sienna behind a Toyota Corolla at a traffic light. As I held my foot on the brake, I began to wonder if the Corolla brake pedal was the same as mine, and thought it would make economic sense for Toyota if it were. I know the same engines sometimes appear in different models, but overall, what percentage of parts is shared between cars from the same manufacturer, and what are the most commonly shared parts? Is that front brake caliper on my Lexus the same one that slows down a Camry? Is the brake-fluid reservoir found under the hood of my Acura the same one that is tucked away at the back of the engine compartment of a Honda Accord? I can't imagine that every part is unique to every car model.

-- John

TOM: I don't happen to know if your brake pedal is the same as the Corolla's. There's not as much room in the Corolla's foot well as in the Sienna, so maybe not. But I know the brake pedals for the Camry and the Sienna are the same part. And that same pedal fits the Avalon, Solara and maybe other Toyotas.

RAY: So, in general, you're absolutely right. There are lots of shared parts for exactly the reason you state: Because it makes good economic sense.

TOM: Obviously, if Ford designs and engineers one directional switch and orders 10 bazillion of them, each one is going to be cheaper (in manufacturing and spread-out engineering costs) than if Ford created and ordered 20,000 of type A, 30,000 of type B and 80,000 of type C.

RAY: It makes repair easier, too, because the dealer has fewer parts to keep in stock, and will be more likely to have the part you need when you need it.

TOM: Finally, it reduces manufacturing costs. When cars are built from the same "platform" (that's the industry term for common chassis architecture, engines and transmissions), several seemingly different models can be built on the same assembly line, or in the same factory.

RAY: Production can then be more easily shifted from slow-selling dogs to the hot models people want. And new body styles can be brought to showrooms faster, because the basic elements of the car are already designed and ready to go.

TOM: So this is the name of the game in car manufacturing today: reducing the number of vehicle platforms used, and increasing the number of shared parts.

RAY: Take Volkswagen, for instance. It recently started using a small car platform it calls "MQB" (that stands for "modular" and then two German words we can't pronounce). That platform is the basis of the Golf and Audi A3, and will eventually be used for the Jetta and Passat, and many others. In fact, VW says more than 40 models (including models VW sells via its other brands) will be based on this one platform.

TOM: So, shared platforms account for the biggest, most expensive shared parts. But part sharing certainly goes farther than that. Switchgear often is shared among different models. So the directional and cruise-control switches, the electric window switches, door locks, light switches, foot pedals and seat-belt latches all are the same. Things you don't see, like ventilation systems, air bags and electrical infrastructure often are shared, too.

RAY: So, what's different? Well, the stuff customers can easily see and feel. The body styles. The appearance of the interior. The level of luxury you experience, like seating surfaces and the amount of leather versus plastic. The amount of sound insulation stuffed between you and the outside world.

TOM: The suspension characteristics can be tweaked to make one model feel more sporty -- the way I like it -- and another model feel more like a four-wheeled living room, which is my brother's preference.

RAY: Actually, I prefer my living room without wheels.