I normally fill up from the quarter mark, on the occasions I don't I notice from the first half of the tank I'll get about 350 miles, then maybe 200 at a stretch til the light comes on. Normally I get around 38mpg, my driving is normally about 60:40 privateroad:town but the privateroad driving is nearly always in the early hours of the morning, so I generally sit steady at a ton rather than 70. I generally accelerate with quite a lot of throttle, and normally use 80% of each gear, changing up around 4,000 rpm (5,000 redline). I've done a "steady" tank and got 45mpg, only a 7mpg increase and thats driving at granny speeds... May as well just boot it all the time for how much little extra it costs me
Fuel gauge is definitely not right though.