Its crazy how everyone differs so much. My dash reads as much as 23mpg on freeway trips with the cruise control set. The best I have calculated is 17mpg which im pleasantly suprised about. Stock SCSB with 373's.
The 23 is what you’re getting as an average on the highway over what ever the distance it was figured on.
I have a Scangage add on it has a instance update readout, on a downhill totally off the throttle it will post up 9,999MPG for the motor has all the fuel cut off.
On an uphill a good one (6%) pulling my 6K trailer it will post up 5MPG more or less.
After several fill ups and adjusting what the Scangage thinks the total fuel used have found it is pretty well adjusted and the trip feature will display a some what close MPG for the trip.
The only way to get a true number is by filling up to the neck opening and doing hand division for MPGs and this will vary on the highway and city stop and go driving.
My best with my 2007 automatic 2 wheel drive on highway only has been 33+MPG around town this time of year is 16.5MPG
One of the call ups I can display is GPH Gallons Per Hour. In gear with the air on shows .72 GPH and taking the MPH that is 0MPG.
But what ever system one has it will work it all into what ever is displayed and yes the Scangage will show 0MPG, stopped and running.
One way to get a good feeling what you are getting to go set up a fuel log at a site like gasbuddy.com like I use for every fill and it will at the end of year will show you totals, gallons, dollars and MPG for the year.