Rule of thumb #'s are a broad way of saying "guess".
The nmpg #'s I've come up with since late 1950's when dad and I calced nmpg together... up to this very point in life... have always been attained by using miles traveled as compared to gallons of fuel usage from fill-up to fill-up. Pretty simple math.
I have a pair of brand new (still in orig package - purchased at an estate sale for very reasonable price) Flow Scan meters that I
may decide to install. Would be interesting to watch them, but not really necessary IMO. Also, due to fact that our Tolly is a gasser there is a simple "rule of thump" I do like to adhere to, and that is; the less fuel line junctions the better! So, although I purchased "new" Flow Scans at such a great cost my sense of "gas line safety" may preclude ever installing them. May end up selling em at good price to others!
I've gotta ask:
In following quote from your post above... are you showing that diesel fuel doubles the btu power per gallon as compared to gasoline... or am I mixed up as to the meaning of the following?
"About 20hp/gallon diesel engine ( or slightly less )
About 10hp/gallon gasoline engine ( .. .. .. .. )"