In my D2 "career" I've had five fuel pumps ranging from genuine to el-cheapo and they've all been totally different. With my current one from full to overflowing reads full for around 200km and then drops like a stone to half full. The red light is a tell tale light that means "the engine has stopped because you ran out of fuel". The first one, before it karked it, had the red light on with about 150km to go.

Now, I just do some sort of mental arithmetic based around 15 litres / 100 km. I look to fill up when my rough numbers add up to about 80 litres used. Of course I get much better than 15l/100km and the tank holds much more that 80 litres but my mental arithmetic should see me right most of the time. If the going is to be rough I use my normal numbers but carry a couple of full jerry cans just in case... I always fill it to nearly overflowing.