spoiler
since there are 8 furlongs to the mile and 4 quarts to the gallon, this is literally just miles per gallon times two
actually fpp is even funnier because pints are almost exclusively used to measure drinks, and it's literally equivalent to miles per gallon because they're both eighths
"furlongs per pint" sounds like total nonsense but it's exactly the same as miles per gallon
so you can literally just substitute it in
@unascribed Or Furlongs per Firkin, if you want to use the mass of the fuel rather than the volume. With pints you're really hand-waving out an additional "at X temperature" factor.