Don't use "E". Use the volume of gas metered by the gas pump and your odometer. Ignore the gas guage, they are generally pretty useless beyond "oh, my tank doesn't have a lot of gas in it".
So, reset the odometer when you fill up. Drive a bunch, fill up again, note how much gas you put in and your odometer reading. Use those numbers to calculate fuel economy.
If it's still less than 20, something's wrong somewhere, either with the car or with your driving habits.
EDIT: by the way, cruise control is generally quite inefficient. If you want to get good efficiency, learn to train your right foot to stay in one spot, roll fast down hills and creep up them. Going up mountainous regions is just going to be very inefficient (dunno if you have mountains where you're at).