Wow, you've got it all set up there. Course we are up to $80 with pump/bike computer/pressure switch.
I really like the bike computer idea, they are like $10 at my local store. Even if it is just fuel consumption and not mpg it can be real useful on a regular route. It could be adapted to optical operation easily enough.
Perhaps, with a "marble", or turbine, or paddlewheel, or?? optical arrangement in the fuel flow, and some fudging with the wheel size adjustment on the bike computer, it could be possible to come up with a useable fuel consumed number (or factor of 10 number) in the distance travelled display (or the current consumption rate in the mph display).
Google check time: the bike computer idea is tried, with a $60 sensor. The second link has a schematic:
http://11109.rapidforum.com/topic=11...6&search=sigma
http://diverse.freepage.de/cgi-bin/f.../fuelmeter.htm