[lindstromjd]

You forget that a HUGE contributor to fuel economy is in how you drive. If you add an overdrive into the equation, you better be doing a lot of sustained, higher-RPM driving. If you do a lot of in-town stuff or sustained low-RPM driving, then you'll probably get worse fuel mileage. The other piece of the puzzle is in your rear end. You have to have the right combo to get good fuel mileage... something the GM techs didn't take into consideration when they gave trucks a 2.72 rear end and a 700r4, and then people wondered why they got such horrible fuel mileage when they weren't on the interstate.