Well, miles till the light comes on, isn't a very good number to use. Maybe run a full tank and record the miles, then do a MPG calculation.
If you figure that you have used 15 gallons when the light comes on, and have traveled 240 miles, then that would be about 16 MPG, assuming that the 15 gallon figure is correct. That's really on the low side.
What are your driving habits, conditions, etc?