Another thing I have been wondering about HP/L and engine efficiency is why is a supposedly "inefficient" such as the 8.4L Viper V10 able to produce 600 hp, yet still manages better average fuel consumption than a 4.3L F430 V8, which only makes 500 hp? The only thing I can imagine is because the engine isn't at such a high RPM when cruising, but is there more to it than just that?
It's all about the ratio of km/h / mph to rpm. Basically, the lower the rpm you can run at a given speed, the better your economy.
Many sports cars are geared short. Extremely short. They're geared so that you hit top speed at redline in top gear. Which gives you cruising rpms that are ridiculously high.
This is because typically high-strung race engines make more power at higher rpms, and allowing them to drop out of those higher rpms makes them feel sluggish... so while a high strung 100 bhp per liter engine
can do 100 mph at just 2000 rpms, it won't feel happy doing it... whereas a large, high-displacement engine will have enough torque to make it feel effortless.
If the engines are both tuned to run stoich (an air-fuel ratio of 14.7:1) as most engines are tuned to do, nowadays, in partial-throttle cruising, and they have the exact same horsepower and gearing, the smaller engine will usually return much better fuel economy at sane cruising speeds.
When looking at EPA numbers, people often complain how some small-engined cars don't get economy as good as big engined cars. The trick is, don't look at the highway numbers... look at the city numbers, as they better reflect real-world consumption... no trick of gearing can get past the fact that a bigger mill will drink more in traffic than a smaller one.