I am assuming that you can normally see the miles left w/o the light on. When the light first comes on you will still be able to see the miles left. When you only have 30 miles remaining, a small car icon w/ a gas nozzle shows. At that point, it does not count down.
I wish it did.
My other MBs did count down to zero (which I liked) but I always pushed the envelope before getting gas and it was not always exactly correct...resulting in calls to friends.
sorry...just re-read your post...to see your mileage at any time...set your display to "trip" and scroll up until you get what you want.
time to read your manual...pg 164