If you are getting less than 18mpg on a tank or two you have a problem, I was only able to get down to 18mpg two different times one was me trying to get a bad tank by driving like an ass all the time. The other tank was me running in SD mode doing some tuning.
What I would look for/do is:
Clean the MAF and TB (remove all sensors first)
Check for vacuum leaks
Replace the fuel filter
Replace the O2 sensor
Replace plugs and wires if they are old
BTW my car is NA so if you are supercharged I would figure the lowest mpg to get would be 16mpg.
