How much fuel a journey uses and what it costs.
Multiply the distance in kilometres by the consumption per hundred kilometres, divide by a hundred and multiply by the price of a litre. A 300 km trip at 7 l/100 km and 1.60 EUR a litre costs 33.60 EUR. There and back is twice that.
The manufacturer's number comes from a laboratory cycle at a steady temperature, with no extra load and no heating. In winter a cold engine, winter tyres and cabin heating add two to three litres per hundred kilometres, and short urban trips push it higher still.
At 5.5 litres of diesel and 1.60 EUR a litre, a kilometre costs 0.088 EUR. A petrol car at 7 litres and the same price costs 0.112 EUR per kilometre. Over ten thousand kilometres a year the gap is about 240 EUR, which is what to weigh against the purchase price.
Rarely. Five cents off a litre on a fifty-litre fill saves 2.50 EUR, but ten extra kilometres there and back already eats about a euro. If the detour is more than twenty kilometres, the saving disappears entirely.