fuel efficieny is essentially the distance ur car can travel in a litre of fuel - petrol or diesel  , fuel efficiency of a car is measured under two road conditions - city & highway

I think there is a diffrence between fuel efficiency and efficasy, i think efficasy will be the distance travelled by the same vechicle for different brands of fuel and for the same quantity.

ders ntn as Efficasy... its efficiency only... which ofcourse can be calculated by Kms burnt divided by fuel consumed (in units)

Hey my engineer friends...... forgot the chapter on engines(petrol, diesel and steam) we studied in school??

Mr. Murari fuel efficasy is defined as the efficiency of the fuel. Different fuels has different efficiency, its the output of the fuel itself which can be calculated by measuring the work done by different fuels used in the same quantity. Petrol is most efficient, followed by diesel, steam etc. Efficasy is inversely propotional to power loss.  

Fuel efficasy is  efficiency of the fuel to burn and give unit energy per unit volume