Dear Sambro,
So every car's fuel consumption is mesured in MPG (Miles Per Gallon). The higher the MPG the more economical your car is to run*. So different engines are more or less fuel effiencient than one another. There tends to be a correlation between the BHP (Break Horse Power) of a car and the engines overall fuel efficiency measure in MPG. for instance if your car has a 1.25 litre engine, with 60BHP the MPG measured on a combined cycle** (average with town and motorway driving) may be 47 MPG, whereas a slightly more powerful 1.4 litre engine with 80BHP may be slightly less fuel efficient and offer 42 MPG. So as a general tenant the larger and/or more powerful the engine the less fuel effiecient it will be.
*One needs to take into consideration though several other factors; engine size and fuel type. So whilst a 2.0 litre engine may return 25 MPG so may a 3.0 litre depending on how hard the manufacturer has worked to implement a fuel efficient engine and/or how much BHP either engines have (in some cases smaller engines on sportier cars have more BHP than those on larger engines). Consequently MPG is only an accurate measure of fuel economy 'relatively' speaking; clearly a 3.0 litre engine with the same MPG as a two litre will cost mroe to fill up as its petrol tank will be larger, however, with the same MPG figure, this just means that it will run for longer but cost more to fill up, but the difference between the cost to fill up and distance a tank can go will be exactly relative to a 2.0 litres. So secondly, it is important to note that diesel engines are more fuel efficient that petrol ones, so a 2.0 litre petrol engine with 150 BHP may return 30 MPG, and a 2.0 litre diesel engine with 150 BHP will likely return 43 MPG even though they are the same size an power.
I shall now continue in a second post as I have reached the word limit and have not yet fully answered the question