Im not exactly sure of how many miles you can squeeze out.
One thing you can do is drive your tank until the light goes off. Then go to the gas station and fill up a full tank.
See how much gas you had to put in and subtract that number from what the capacity of the tank is...i think it's like 70-75 liters or something like 18.5-19.8 gallons.
You can calculate from there how few miles you can squeeze out with your stellar MPG.