Diesel is 38kwH / Gallon - Gasoline is 34 kWh / GallonDiesel is a denser store of energy than gasoline, so there's that too.
That's 10% difference in Energy density - for most Diesel are getting 30% or better fuel economy in the same vehicle
Yes, and if we are to explore exactly why diesels are more fuel efficient we have to include higher compression ratio, unthrottled inlet, lower (mean) temperature combustion, more favourable ratio of specific heat during expansion, extended expansion ratio due to turbocharging (to the benefit of reduced 'air work' on the inlet side). It is less that low rpm delivers good fuel efficiency for them, more that low rpm is all that they are capable of!
Dr Diesel threw himself overboard. A pity he didn't take his engines with him.
All those measures are done to keep the RPM low. There had been experiments with Diesel which had throttles, there had been diesel with low compression to get to higher RPM. From a chemistry perspective - Diesel burns slower, The oxidation process is slower- the flame front is slower - there is a physical limitation how fast your can rev it because of those factors. That's why you optimize a Diesel engine to produce most of it's output low and revs - means less parasitic losses from the rotating assembly.