Most gas milage related issue will be cause by a worn-out O2 sensor. The l67 has two O2 sensor. the manifold sensor is primary use for fuel consumsion, and the second O2 sensor at the cat convertor is to let the ecm know the cat convertor is performing correctly. also try a throttle body cleaner and clean the maf sensor element and around the throttle blade. the gm Maf sensor has the ability to burn off any dirt which can get stuck to the sensor element. yea its buning off but i am sure the ashes are still stuck to the element. i have seen it.
even if the car does not have a ses light this does not mean the O2 sensor is performing with the millivolt range, .100mv above the 14.7 a/r ratio will cause the engine to consume more than 10% excess fuel. You can find all this out by reading the spark plugs or a scan tool. if your current O2 sensor has over 60,000 miles, then i will highly recommend replaceing them with bosch,ac delco or performance application.
the EVAP system will cause hi fuel consumsion, but you don`t know a ses light so i will say the EVAP system is functioning ok. you can also perform a injector leak dowe test, if you smell gas or exhaust gas while driving.
check the tire pressure too, it getting cold.
hope this help.
