I have a petrol Peugeot 308 auto - 130bhp.
On a long motorway journey at ~65mph cruising speed it will comfortably exceed 50mpg.
Around town with mostly short journeys - a little over 30mpg
A manual will probably do a little better. A larger, heavier car will do worse. SUV or 4X4 will be worst of all due to aerodynamic efficiency being closer to a brick than arrow.
Engine size is largely irrelevant - often a larger more powerful engine with higher gearing may be more economical than the smaller.
Energy used is a function of weight, aerodynamics, friction, hills, acceleration, use of brakes, driving style.
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