There are three different approaches that manufacturers take when trying to develop the most economical car.
They can use a standard gasoline or petrol engine. This has historically been the favoured approach simply because there was no alternative until relatively recently.
The second choice is a diesel engine. Diesel technology has improved immensely over the last two decades, becoming highly efficient and, importantly, very refined.
The final approach open to car makers is the gasoline or petrol electric hybrid. This technology has only been around in the last decade and consists of two powerplants in the one vehicle - a gasoline engine and an electric motor.