When choosing a new car, there are six key elements which will have the biggest impact on fuel economy of any particular vehicle:
Size and weight of vehicle
The heavier and bulkier a car, the more fuel will be used to power it.
Size of engine
Generally speaking, smaller engines are more fuel efficient than larger engines (often at the expense of power). However, this isn’t always the case, as simply choosing the smallest engine in a range may mean a car is underpowered, meaning the engine has to work a lot harder to power the car, hence using more fuel and also increasing wear and tear on the engine. The most fuel efficient cars have a good balance between engine size and vehicle size and weight.
Type of engine
Different manufacturer’s engines will vary in efficiency, upgrades to engines in the same make and model of car will affect efficiency and different types of engine, such as petrol/electric hybrids, will also have an effect on fuel economy.
Type of Fuel
Diesel engines are usually more fuel efficient than their petrol counterparts (based on the same size engine), and turbos and other sources of increasing power in an engine will affect fuel economy too. It is worth noting that if there is a relatively large gap between diesel and petrol prices, it is not always more frugal to purchase a car with a diesel engine, since low yearly mileage drivers can actively be better off purchasing a petrol engine over a diesel equivalent.
Generally, automatic transmission vehicles use more fuel than manual transmissions, although new technologies in automatic transmissions (such as dual clutches) are reducing this gap all the time, and the difference is now almost negligible (especially in larger vehicles).
Aerodynamics of vehicle
Aerodynamics play a big part in a car’s fuel consumption, since the less drag or wind resistance a car has, the better its fuel economy will be.
Age and model year of the vehicle
Slight variations in what appears to be the same engine can occur from model year to model year of vehicle, so it is worth checking the model year of your next car before purchasing, as if an engine has been improved slightly, you may find better MPG figures in the newer model year car. This can vary quite dramatically if you're comparing two identical vehicles several years apart in age.