Here's the Yanmar L100 fuel graph.
![Image](http://i301.photobucket.com/albums/nn69/DaveWarham/Screen%20Shot%202015-06-17%20at%2010.32.31_zpsq9nffjcl.png)
50 mph for old Sluggy is about 2700 rpm so spot on the curve for best fuel consumption:
Here's the maths bit - it's MPG so apologies for out continental members who work in Litres per 100km
g per Hp hour = 180
50mph needs 8hp (ok this is a best guesstimate)
So to travel 50 miles you need 8 x 180g = 1440g or 1.44kg of fuel
Diesel weighs 3.87kg per Imp gallon (now I've lost out US members too)
So 1.44kg / 3.87kg = 0.3721 gals/hour
So 1 gallon will take you 50/0.3721 = 134.37 miles - Ta Da!
Now that's pretty close to what I'm actually getting.
Now moving up the theoretical chain
3600 rpm for me is 66.65 mph at which point we are up to 200 g per HP Hour.
If we accept that 10hp will power you to 66mph..... then MPG dips down to 97 - quite a reduction.