The declared fuel consumption of my new Renault Kangoo Maxi van is 4.8L /100km, but in reality it is impossible to get under 8.0L/100 km. Renault garage staff have tested the vehicle and "say" that everything is in order, and that their fuel numbers were around 5.8L/100km (City). Is there any sort of engine or other problem that could cause this astronomical discrepancy in fuel consumption?
There are things that could cause abnormally high fuel consumption, like a faulty oxygen sensor, faulty ECU, blocked exhaust, but the dealer mechanics found no such problems when they checked your car. The fuel consumption figures quoted by carmakers is the result from a laboratory test under controlled conditions, it's meant to provide a means of comparing makes and models, rather than give you a real life figure that you could expect to get from your car. That said, the difference between the test result and your car's consumption is too much. I would sit down with the dealer and have them explain the reasons for the difference.