I’m not sure of 30%, mostly I’ve heard 10 to 20% with BioDiesel being less.
Part of the benefit of Diesels is a higher CR and no throttling losses.
Having a m/t allows easy coasting and even manual engine stopping in the City.
I recently bought a new Turbo gas vehicle similar to my brace of Turbo Diesels.
The Diesels win the real world FE test hands down.
The new veh does have AWD, so there is that.
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.