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.