The effect of horsepower on gas mileage

[Matt Timion]

I've heard this many times, and I can't confirm it. Please help.

When I first started planning to increase the fuel efficiency of my car, some people said that race builds and economy builds are almost identical. Their theory behind this was that more horsepower means the engine works less to get to a certain speed, and this in turn saves on gas.

Made sense to me.... sort of.

I mean, if I threw a turbo in my car, or made some other modification that gave me an extra 30 HP, would that mean I could travel the same speed with lower RPMs? It seems to me that is the same way the long geared transmissions work, which give very high gas mileage.

So it makes sense, if it's true, except for the fact that the guys with super "fast and furious" cars only get like 20mpg. It can't entirely be due to the transmission.

Some people have claimed that higher HP equals higher gas mileage for this very reason. it is the exact thinking behind advertisements on Cold Air Intakes, racing headers (exhaust manifolds), and other performance modifications.

So, is this true?

If so, what is to stop me from getting a junked up CRX (very light) and doing the Individual Throttle Body conversion? (the ITB conversion will use a throttle body from a Honda motorcycle and use it for each cylinder, allowing for more percise airflow and claims to increase HP by 30). Will such a modification give me better gas mileage?

And lastly, if more HP means better mileage, why do all of the gas mileage beasts (prius, insight, CRX HF, Geo Metro, etc.) all have tiny little engines with no power?