I believe you are "barking up the wrong tree"
but the best 'real world'
BTU per gallon information I know of is this official EPA study from around the country:
http://www.epa.gov/orcdizux/rfgecon.htmThe Bosch Automotive Handbook has tables of both gaseous and liquid fuel heating values.
as a historical aside,
the story of the invention of calcium carbide
for miner's lights
'blue tipped wrenches'
Union Carbide Corporation
Mr J. M. Morehead,
the Morehead Scholarships at UNC-Chapel Hill
and the Morehead Planetarium there
are all worth reading about:
http://en.wikipedia.org/wiki/John_Motley_Morehead_III