Played a little game of fuel light roulette a couple weeks ago in the 328. The light comes on when the needle gets down to the top of the red zone. Once below 20 miles range it goes to ---. The gauge goes all the way down to 0 then no further but keeps on going. Test mode on the OBC gives a readout to the tenth of a litre remaining. Took this all the way to 0.0 and maybe half a mile to the petrol station after that. I'd say this is pretty close to empty, it was misfiring a bit at this point, put 60.0l into it.