F-150---- V8 Gas Mileage? What MPG Do You Get??

I've now had this truck for 4 months.
The very best average mileage I've gotten is
16.0- Miles a gallon. I mean that's the best..
That's highway doing the speed limit
with cruise control set.
For the most part I average around
14.5 MPG combined city and highway.
Do I have a problem??
Trying to find out what others driving a V-8 are getting?
I did have an O/2 sensor go out at 800 miles,
So do I have another problem?
Appreciate any feed back I can get..
Thanks.....