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Maybe try a fuel-flow meter?

I have this one and it seems to work pretty darn well:

http://www.standardhorizon.com/indexV...visionID=3

EDIT: I should have mentioned that you can set the above instrument to show remaining fuel in the tank, not just current fuel flow/usage. When we fill up the tank, we reset the amount of fuel and it just counts down remaining fuel as we use it.