In reality, what units are you using? Did you mean to say 18.5, which is typically 7-10mpg below what most see, with some able to still get in the 30's at a regular cruise. You might need to scan what the fuel trims are doing while driving, and monitor various air and fuel parameters to get an idea what may be causing it to be lower than normal.
A forum community dedicated to all General Motors makes and models owners and enthusiasts running the 3800 series engine. Come join the discussion about performance, modifications, classifieds, troubleshooting, builds, tunes, superchargers, maintenance, and more!