From their website, (
http://www.scangauge.com/features/tripcomputer.shtml) it does sport the functions you would want: trip economy and actual used.
I myself do not own one of these tools, I only know about it because I am in the market for a can-bus compatible scanner. I cannot vouch for it's accuracy. Based on the threads on different forums, scion and miata I believe, they swear by it.
To me it also feels like a toy, mainly because of the digital guages which I would probably never use. An excuse around this is this product is a fully functioning trip computer and a engine code reader.
I would probably pick one of these up for my new Versa, but that comes after a set of winter tires as I do go north to Blue Mountain to ski and what not. Add to the fact that I just emptied my bank account buying the Versa and my desire to buy a set of skiboards and the mods I would want for the Versa... it would be well into the year before I do get one.
Oh, and for the fueling problems, certain pumps make my Mazda 6 click way too prematurely, namely the Esso stations in Toronto. I would just keep in mind how much fuel that car would need based on what the gas gauge says. Not that this would help with your calculations one bit.