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A transmission with overdrive increases gas mileage and reduces engine...

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Dear Tom and Ray:


A transmission with overdrive increases gas mileage and reduces engine wear, right? So how come every car doesn't come with overdrive?
Ken

TOM: Almost all cars do, Ken. Except for a few remaining cheapo cars with three speed automatic transmissions, almost every car sold today has a transmission with overdrive.

RAY: Overdrive is not some mystical device. It's just a gear which allows the wheels to turn faster than the engine.

TOM: In the old days, the lowest gear ratio (the "highest" gear) used to be 1:1. That meant for every one revolution of the engine's crankshaft, the driveshaft (which ultimately turns the wheels) would also revolve once. But then, one day, some engineer was sitting on the porcelain throne and had an epiphany. He realized that there's no reason that the driveshaft couldn't turn MORE than once for every revolution of the crankshaft. "Eureka!" he cried, as he jumped up to tell his colleagues. Well, the rest of the story gets a little messy, but suffice it to say that the idea of overdrive was born.

RAY: So any gear that has a ratio lower than 1:1 is called an overdrive gear. It's usually fifth gear on standard transmissions, and fourth gear on automatics.

TOM: And it does exactly what you say it does, Ken. It improves gas mileage and reduces wear and tear on the engine, because it allows the the engine to turn slower at highway speed. Most engines used to turn at about 3,000 rpm at 60 mph. Now, some engines on cars with overdrive turn at half that speed! And you can do that because it doesn't take much power to KEEP a car going at highway speed once it's already AT highway speed.

RAY: And that reduction in engine speed makes a big difference to fuel economy. So I predict we'll see cars go the same way as bicycles.

TOM: Right. We used to have one speed bicycles. Last time I checked, there were 21 speed bicycles. Similarly, we're seeing six-speed manual transmissions now and five speed automatics. And I think we're going to see more and more of that because it's a relatively inexpensive way to improve fuel economy.
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