I too am having the fuel mileage issue and I have been fighting with GM for months. I have had my truck at the dealer to do "diagnostics" but without a check engine light everything is now within the GM specs. I lost about 25% fuel mileage and I am not letting this go with GM. I was told to contact the NTS to get a recall done on the recall. The emission recall should have made the truck more efficient but obviously when you lose 25% the truck is running less efficient. With enough people we can probably get a class action law suit going and force GM to correct this.