[SteveBrass]

The miles til empty reading is just that; it doesn't say how much fuel is actually in the tank, but rather how far you can get on current consumption.

So if you had a gallon in the tank, and had spent the last 5 minutes stuck in 1st/2nd stop/starting it would show not many miles to empty. But if you then drove at 50 in fifth along a level stretch of motorway the miles to empty would increase.