There are so many variables that I doubt you will get many answers.

Modern cars have computers that have many inputs that can allow the engine to use minimum fuel to run at idle, even when the AC kicks in and out or the headlights or other equipment is used..

If I can, I will idle one of our XC 70s on flat ground so that the car moves and I might get a MPG reading.

Older cars depend on carburetors that must be adjusted to allow the engine to idle under all conditions that might arise.

In addition, the carburetor might not be adjusted correctly..

If you look at the inside or a car's tailpipe, some, like our '88 745 GLE might be a nice tan color - due to tiny particles of silicon that made it through the engine.

If your tailpipe is sooty black - your car might be running too rich. Clean the inside of the pipe and look at it again after a hundred miles or so.

BTW - I run my 544 a little bit rich to avoid having it run lean which can lead to engine damage.