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Theoretically you should be able to drive faster and get better fuel economy IF you have unlimited gearing. But a lot of vehicles are limited by their top gear as to whether better aerodynamics and driving faster would help. There's also the question of what the speed limit is.
On the other hand, if you could switch in a less powerfuel but more efficient at lower loads engine and improve the aerodynamics, then you'd really be increasing your fuel efficiency. That or use a pulse and glide technique to get great BSFC loads at lower speeds along with your aerodynamic improvements.
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