The onboard computer will give you an estimate of the Range left on the tank, but it is not very accurate. To see the range, just click the right upper top button on your steering wheel and cycle thru all the small display screens to find it. When I first fill up the tank, it usually will show range of 450 miles or so which is actually about right. But as I drove that range goes down faster than it should. By the time it ranges 0, usually I would be at about 8.5 gallons down, which means I still have about 3 gallons left in the tank and can easily go another 100 miles.
I use range to give me an idea when to fill up. When it reaches 0, I will start to think about filling up.