Question

what is the most economical speed of a Passenger car

Answer 1

The most efficient speed for electric cars is closer to 15-20 mph versus 55 mph for ordinary cars. This is because the losses at low speeds for ordinary cars has to do with the construction and inefficiency of the gas ICE. Directly to the question at hand - they also give the sources of losses over the range of speeds.

Answer 2

40 Ya 50 Ke as pas hota hai speed