jeremy, fantastic explanation. i wonder if you might know if this might solve my problem. i have an e200 and the other day i put 50 euros in it, first time in a long time, and gauge went upto 3/4. on taking off, gauge fluctuated up and down, but never going below 3/4 and the reserve light would come on at speed. now it appears stuck at 3/4, obviously until i turn the engine off. is it possible that something got in the sending unit while filling up and that has caused the gauge to stick and the light to come on intermittently?

thanks in anticipation, and hopefully i'll be able to learn and contribute here in the future

cheers dave