What is better on gas mileage gas or diesel?
Diesels almost always get better mileage than gas engines, some like the Volkswagen diesel are very efficient. The fuel is quite a bit more expensive though so do some math first before buying. Compare the mileage estimate of the diesel and the price per gallon of fuel against the same data for the gas. The best solution would be a gas electric hybrid.
Some cars have diesel engines because diesel engines give more mileage and diesel is cheaper than gasoline in some countries. In the UK diesel fuel costs more than gas but, because diesel engines give much better mileage (miles per gallon), people are still happy to pay more for the diesel fuel if they run diesel vehicles. Diesel engines last much longer and have much better torque at low revs than a petrol engine.