Is the GM specs as to fuel mileage including if the electric was also used and do they spec if not used ?
As one mentioned, mileage depends on how much of that crap Ethanol percentage is
To this day, most people have no clue that the feds increased max Ethanol from 10 to 15%
Higher it is the less fuel mileage is, and less performance engine output is, so it is under more load and use
When the feds first mandated this 2 years ago, the station pumps had to label if 10 or 15%
People seeing E15 than instead moved to pumps saying E10
This pissed off the Ethanol business and lobbied feds to remove a rule so that no Ethanol stickers
were required on station pumps at all
IF in doubt, always ask a station what the percentage is before pumping into your gas tanks.
Fill up with E10 and see what the mileage if like for a couple of tanks
Then switch to E15 and test that for MPG
Ethanol eats tank, fuel system internals,
it has screwed with the fuel float sensors causing incorrect reading of how much fuel is really in the tanks
since the early 2000s
Test with a known full tank.
When gas gauge says half tank,
Then fill tank and see how many gallons it took and does that match the GM spec as to tank side.
The more Ethanol percentage is, the leaner the AFR is, the more engine knock, less timing and all
that costs fuel mileage
Check with a OBD-II scanner and see what the fuel trims and O2 sensors report
Maybe need to do an engine decarb, clean injectors, MAF, throttle body and assure clean
And that there is no airleaks causing leaner AFR which would use more gas.