Not sure it is 40% by the time you allow for the much lower HP.
Somebody with a brain and decent training could fly any old PA28 and get a 30% fuel saving off the fuel flow which a typical PPL will be getting, just by correct leaning. And by doing a lot more stuff like gentle cruise climbs rather than doing all climbs full-bore one could save more.
There is still an "MPG" saving with a diesel but it's nothing like 40%,
AFAIK.
Most of the case for diesels is currently from the tax difference.
How come there are no more 50hr checks on the airframe?