[m55au]

I looked around a bit in "definitions.units" and there are actually energy densities in there for various fuel sources, including diesel.

  You have: 1 diesel
  You want: Wh/L
   1 diesel = 10103.465 Wh/L

Slightly different value than in Wikipedia, but close enough.

[dredmorbius]

*blink*

TIL, there are!

Thanks.

(For the curious, the definitions file is ~7500 lines, there's a lot in there. GNU version. For those on MacOS, the stock BSD units offers far less.)