How Is Economical Speed Calculated for Fuel Efficiency?

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A-Kim
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This is my first writing in forum. And I'm not good at English... Please understand my poor writing.

Resistance force : F=kv2 (N)
Base puissance : P (W)
Time : t

Fuel-efficiency : e (m/J)
Economical Speed is fuel-efficient speed for a car. (e is maximum)

Energy for resistance force must be kv3t
Energy for base puissance mush be Pt
Fuel-efficiency e must be vt/(kv3t+Pt) = v/(P+kv3)

Is there something I have to consider more??
Or please suggest some other method to get economical speed.
 
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I don't think you can get fuel efficiency from this setting of the problem.
The force from the engine (the one associated with power) should be equal to the resistance force, for motion with constant speed. You don't use energy for both of them so there is no reason to add them in calculating the work done.

Fuel efficiency depends on the details of engine, not just on the expression for resistance force.
 
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