One of the big assets of the diesel is at idle. A diesel only has to inject the amount of fuel into the cylinder to get the engine to idle at its targeted idle speed. Without the A/c running and a bunch of electrical load that can be very low.
A gasoline engine has to maintain a stoichiometric ratio (fuel and air) to run properly. Too lean and it burns itself up.
Too rich it fouls itself and the catalytic convertor. The extra energy goes out the radiator and exhaust.
A diesel will cool down significantly at idle.
“no throttling losses”
Not asked, but answered...
Electronic Decel fuel shutoff is awesome, even with Diesels.
Injecting/Burning NO fuel is quite efficient.
The a/f ratio is important too.
I tend to shut off all vehicles at a light if I know the cycle is longer than 60 seconds.
That's why my Diesels have heated seats.
If you don't get above 30 mph in the Winter, the thermostat will NEVER open.