I'd skip this one. Calculating fuel consumption just isn't that hard. And this thing is designed to gauge human water consumption, which would be considerably less than a cars fuel consumption. At least my car. And if it failed from fuel pressure that it's not designed for, and your fuel pump kept running it would spray fuel all over wherever you decided to install this thing.
Even if your car didn't burst into flame, your mpgs would be really lousy from all the wasted fuel you sprayed all over. Not to mention the pollution...
Nah, I'll skip this one.