but the light comes on with about 1/8th of a tank showing so that means nothing... All I can say is that I can drive about 50 miles from when the light comes on, the needle goes waaay past empty (im talking about another 1/10th) and if I put £30 it will take it to a 1/3rd
If in the same scenario you are only getting the fuel light to go off on the "cusp" then there is something wrong with the sender/guage....
You are confusing me mate. The OP wanted to know why his lite was still on after putting in 30 quids worth of gas. I am purely demonstrating (through the maths) that if he was literally on fumes, £30 may not be enough to "definitely" put the light out. So fill up and see if the light goes off, before spending a pot load of money on a problem that may not be there. 10th's, 8th's, qtrs,thirds of a tank make no difference, the physics of fuel fitting in the volume of the tank at a given cost per unit do make a difference.
If the OP was running on fumes 10 litres of fuel would be enough to extinguish his low level light, it might come on again with in 5 miles or so but defo would not need anywhere near £30, and thats without doing the maths.