I have an MPGuino for my car. Not
in the car, but at least it's built...
You hook it up by hacking into the engine management system's wiring harness. Well, hopefully you "carefully and tastefully connect it into" the harness. It needs to be connected to one of the fuel injector signal wires, the VSS wire (on our cars that comes from the instrument cluster), and switched power and ground. I think that's it. You have to find which wire is what, and tap into them. I'm still dithering about exactly how I want to do that...
The fuel gauge in my current CRX and its 1990 Si predecessor is very nonlinear. I can go over 100 miles in the top 10% of the reading on the gauge. The next 100 will take me to the halfway mark on the gauge. The next 100 miles will put me just above the empty mark, and by the end of the last 100 miles the needle on the gauge will no longer even be touching the empty mark. And then it'll take about 9.5 gallons (in my car, that's out of about 11) to the auto shut-off of the gas station's pump.
Oh, and all of my Hondas would have gauges that read lower when first starting the car in the morning, and would read higher after driving around for a while.
-soD