i once thought i was getting 9-10mpgs until i realized with my 35's for every 100 miles im actually traveling a little over 120 miles and did the math and my mpgs were actually around 12-15mpgs if your speedometer is not re calibrated via programmer or speedometer gears it will throw you off at first and make you think you are getting crappy (er) gas mileage than you really are.

for example say my odometer says i go 300 miles but in reality i travel 360 miles and divide that by how much gas i use say 24gallons which would be a full tank for me i divide 360 by 24 and its 15mpg give or take.
i only get this kind of mpg when im cruisin on the freeway not goin up hills or anything. in town i get about 10 to 12

 

Have your O2 sensor manually checked. It can be bad enough to make it run like that,but not be bad enough to throw a code.

Symptoms are poor mileage, and funky smelling exhaust.

Been there,done that with my S-10. Tried everything I could find. Lucky my mechanic knew enough not to trust them to throw a code.