"Made like a gun, goes like a bullet"
So many Enfield stories. So little time!
Shutting off and coasting downhill might have helped. But probably, yes, going slowly would be a big help. Naked motorcycles have poor coefficients of drag and so should benefit more from the low wind resistance of low speed than, say, a Prius. Also -- who measured gas usage? A factory representative. Not implying that anyone cheated but there's always some rounding off to be done at an informal event like this, tanks can't be bone dry, a pump may not meter exactly, etc. Everyone wants a good result, which spirit contributes to lack of challenge when some factor is overlooked. Really just a fun challenge.