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Old 06-29-2012, 08:08 AM
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floydarogers floydarogers is offline
Pedant and Curmudgeon
Location: Renton, WA
Join Date: Oct 2010
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Mein Auto: 335d, 328d, Toyota T100
Originally Posted by bmw_or_audi View Post
You are not going to see big improvements in city mpg. In city driving, you are accelerating often from low speed, meaning you are mostly fighting the car's weight. Turbo or not won't make a huge difference. But during freeway cruising, you are barely using 20-30Hp (guessing). At these low power levels, the internal resistance of the engine becomes relatively more important, and the small displacement gives you a clear advantage.

So basically, when you are generating a lot of power, the engine's internal resistance robs a relatively small percentage of power, so small displacement buys you very little. When you are generating low power, such as when cruising at legal speed limits, internal resistance becomes more important in terms of power, so small displacement gets you noticeably more efficiency.
That is a pretty good explanation. Hybrid powertrains get better mileage in city precisely because the gas engine doesn't have to constantly "fight" to re-accelerate back to speed. And on the highway, those tiny gas engines are perfectly capable of keeping things moving.
It's also why small diesels get better highway mileage than hybrids.
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