When does it become necessary to wash the fuel tank of a car? I ask this because I know that the fuel we buy does contain some dirt. I have noticed some of this when emptying a jerry-can of petrol into my generator.

I have used my car for more than four years now, so I keep wondering whether there is already a sediment of dirt in my tank that I need to get rid of.

Thats the reason the fuel pump has a filter and also the filter to the lines to the fuel rail

The only time i'll recommend "washing" ( i assume you mean emptying the whole content of the tank ) is when you buy adulterated fuel, there was a time in Port Hacourt, Woji to be precise, where after a rainstorm, water got into the underground fuel tank of a petrol station and they sold the fuel-water mix to unsuspecting motorist. come and see commotion! At least 10 cars were ground after buying the fuel, till it dawned on us that the fuel was the culprit and the angry motorist nearly lynched the station manager, especially after he refuse to accept responsibility for the damages, if not for some policemen station nearby. That episode cost me a fuel pump to put my ride in order.

So for me if your fuel is okay, i don't see any reason to "wash" your tank.

Only wash it when you feel its dirty.