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I just found this thread from the (monthly?) email and figured I could contribute; I've not read the entire thread as there's alot to take in (more than I have time for at the moment) but as a guy who owns a turbocharged gasoline (e85 really) powered vehicle (with another being built) I figure I could add some value to the thread.
A few things to note here:
1. Turbos require a specific volume of exhaust gasses to function; a volume you won't get at cruise with anything that isn't a diesel.
2. It takes a certain amount of horsepower to sustain a constant speed; basically whatever it takes to overcome things like wind and rolling resistance and internal engine friction, etc.
3. It takes a certain amount of fuel to make that horsepower.
4 Gasoline engines do not generate boost during cruise, unless going up a steep grade. In fact, intake manifold pressure should be very, very similar to an all motor/NA vehicle.
5. Improving the thermal efficiency of the engine (less fuel per HP) doesn't really have anything to do with turbos (in fact, adding a turbo will require a fuel flow per HP increase over N/A vehicles; ex: BSFC)
6. If memory serves, fuel consumption testing is done with "typical driving scenarios" which don't include heavy throttle application; this suggests that most turbocharged engines will not see much, if any positive manifold pressure from their turbos during this testing.
Again, apologies if any of this was already covered, but if it was, it's worth reinforcing.
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