Not really, hvac. As velocity doubles the amount of power required to overcome aerodynamic drag increases by 8 times. While I acknowledge what you say, there is an increase in power and efficiency from 2300rpm to 2700rpm, in a diesel that increase is actually quite small since they have such a flat torque curve. A petrol might be a better example but even that would not have an increase of 8 times, required just to overcome drag, not actually increase efficiency to the point where significant fuel savings were being achieved.
In aviation there is such a thing as the "sweet spot" where drag is minimised at a certain speed before increasing dramatically up to about Mach 1 from which it actually starts to decrease. If our Tritons could do Mach 1 I don't think we'd be having quite this conversation.
I do understand, though, that for most cars that sweet spot where fuel efficiency vs drag are at the optimum is around 80km/h. Varies, obviously, according to design and accessories.