Fuel consumption and fuel efficiency work in direct relation with the other. Fuel efficiency is dependent on the fuel consumption of the vehicle driven and the distance driven and the type of vehicle. More to the point thermal efficiency is the conversion of the vehicles fuel as it works. This is the efficiency of the fuel. This varies from vehicle to vehicle, depending on the type of vehicle, the method of use, etc. The fuel economy of the vehicle also derives the calculation from the distance driven and the speed of travel. A better vehicle will drive further on less fuel and this type of vehicle has better fuel efficiency. In addition, because it has better fuel efficiency it will have less fuel consumption.
To put it plainer, there is a difference between fuel consumption / fuel efficiency. The fuel amount used (litres) per 100 kilometres gives the consumption. For example, a good fuel economy will have a lower number. One that has a higher number is a vehicle that has poor fuel economy or efficiency. Therefore, fuel efficiency is the amount of distance traveled per unit of fuel used. For fuel efficiency you want to aim for a high number because the higher the number the better the fuel economy. The lower the number of fuel efficiency the poorer the economy of the vehicle.
Different makes of cars have different fuel efficiency and therefore different fuel consumption. For example, American made vehicles generally are not as fuel efficient as European or Japanese manufactured vehicles. The type of driving will affect fuel efficiency. For example, a person living in a large city, who has to stop and go a lot will have lower fuel economy than a person who can drive for kilometres on the open highway. The higher speeds will pull down the fuel economy while driving the safer recommended (lawful) speeds seem to offer the best fuel economy.
Petrol engines have less fuel efficiency than diesel engines, normally more than ten percent better. A person can typically drive a vehicle with a diesel engine about 25% more kilometres per litre. However, these days there are more petrol or gasoline engines that have better fuel economy than they did in years past. Diesel fuel yields higher energy output per use than gasoline, meaning more of it is useful to the energy of running the vehicle and not wasted.