Thank you, Passerby2, for the link.
I read the reasons why one gets poor mileage, like the veritable "lead foot" syndrome. It helps to remember that the brake pedal should be used in moderation (by looking ahead and anticipating slow downs ahead) since the acceleration is paid for with the gas pedal, it helps sometimes to just remove one's foot from the gas pedal and ---coast. Only brake when you absolutely have to do it!
IMHO,
Perhaps the most salient reason for bad gas mileage is the engine timing. A lot of people will buy the cheapest gasoline, regular, when the engine actually needs Premium High Octane. A knock sensor can retard the timing, but this means that the spark will come too late for maxiumum power (and fuel economy).
In the old days of motoring, the general public thought that the gas and air make an "explosion" but now more people understand that the gas and air burn at a constant rate. This is the reason that motors have an initial static timing before Top Dead Center, BTDC.
This early ignition allows the pressure inside the cylinder to grow so that the maximum down force on the piston occurs at about 15 to 18 degress after top dead center and this force accelerates the piston down to the bottom of the cylinder. At Bottom Dead Center the piston is done creating power and the rest of the pressure is vented via the exhaust valve. Hopefully, the heat and pressure are captured by the turbocharger for some boost, but in naturally aspirated engines, the heat and pressure are routed to the catalytic converter and the muffler and are gone.
One doesn't need a performance exhaust to understand how a cloggged cat or a restrictive muffler can ruin your gas mileage. A good muffler shop can actually check the back pressure of the system by drilling a small hole and attaching a gauge.
Also a good tip is to check the sparkplugs, especially since the new cars have a lot of carbon from the EGR system and the GDI (Gas-Direct-Injection) that can clog the intake manifold and the intake valves.