I've gotten has high as 19mpg with my V8 and sometimes even single digit fuel economy. When I'm doing lots of short trips around town, mileage plummets, especially in the winter with the colder, denser air, winter-blend fuel, lower tire pressures (until I fill them back up) and thicker lubricants in the crankcase, transmission, and differentials.
I noticed an improvement when I switched both differentials to Mobil 1 LS gear lube. I also run all four tires at 35psi. Going much below 30psi tends to kill my mileage. I tend to get the best fuel economy in late spring and early fall, when I drive about 60mph. Going over 70 kills things (speed limits out here are 75 on the interstate).
Driving style also plays a huge role in fuel economy. Even "normal" driving isn't the best for fuel economy.
Ethanol-blended fuels tend to hurt fuel economy too. Out here, I still have the choice of 100% gas or 10% blend (E10). Some states do not give you that option. Given the choice, I will not run E10 in anything. In the four vehicles I've run it in, I've always gotten reduced fuel economy with it.
In closing, unless you're getting check engine lights, you're 99.9% sure to not be running rich or lean. The ECM monitors and controls fuel mixture. If it gets "out of bounds", you'll throw a code.