Can you describe the intended use a bit more? Is it running in the garage or sitting still, yet somehow needing a refill every two weeks?
If you get the mpguino dialed in for your vehicle it will give you instant, as well as how much it thinks you used for an arbitrary amount of time. I don't know where the other fuel is going in your situation.
You could also weigh the vehicle I suppose.
Also maybe you can tee into the fuel line and add a sight gauge:
