Hello everyone 😊

I have just bought a W220 S500 (2000 model).
I have encountered a problem that I am not able to solve. The cars on board computer states that I am currently using 1,4 litres/10km. But the analog fuel gauge is not agreeing with the onboard computer 😂 It states that the gas tank is almost empty When I have only driven 400km. If the tank capacity is 88 litres I should be able to drive 620 km on a full tank of fuel with the estimated consumption of the cars onboard computer.
I have replaced the spark plugs (Bosch platina plugs), air filters, oil and oil filter (mobil 1 synthetic), but that did not change anything. I have also reset the ecu with no effect.
Does anyone know How I can solve this?

I have to appologize If my written english is not the best. I am Norwegian and I dont use written english as a way of communicating to often 😂