I just joined, and I was hoping to see this very thing. I quickly saw the problem with this, as in my own vehicle class, someone managed to enter 2402 mpg, and not surprisingly, is listed as "Top of garage". I did see a couple of other mpg reports from Matrix owners that are unrealistically high, but only by a few mpg.
This is a great concept, but as often the case, along comes a bonehead to mess with a good thing.
In looking more closely at the records for the Matrix claiming 2402mpg, it appears to be an honest mistake. The user appears to have started using the Odo readings, and then entered one as a trip odo. Seems there should be some sort of error check for this?
In addition to the Insight, another 'king of MPG' is the Audi A2 TDI 1.2 (the so called '3L' model).
There is only one registered on fuelly, but the vehicle is designed to do 3 litres / 100 km (hence the '3L' badge). Sadly only manufactured between 2000-2005, and only sold in a handful of countries. It was ridiculously expensive to manufacture thanks to features like the magnesium alloy wheels.