I have had the Ultra Gauge for several years, can't remember when I bought it. But I think I was one of the first ones that used it on a 2003 D2, as it had some problems and the company wrote a modification to the firmware to make it work properly.
Yeah, plug it in, wait a few minutes for it to figure out what sensors are there, etc. You can change which statistics you want to display.
I do not recall what I set for tank size, but I think I used the capacity in the manual. That said, I use the gauge on the dash to tell me when to go fill up the tank, not the UltraGauge readout.
I calibrated the fuel usage and the distance. But, I found that city driving with lots of cold starts results in a lot of error for fuel usage, especially in winter. That could give you a very overly optimistic estimate for distance you can travel on your remaining fuel.
I do not know how it calculates fuel usage, but I suspect (could be wrong) that it reads an air flow into the engine senor and then uses a generic stoichiometric value for air to fuel ratio to estimate fuel usage. That is great if your engine is always warmed up and everything else is at peak efficiency. But that will give large errors to fuel usage when the engine is cold, so lots of short distance engine starts would result in more error. I am retired, rarely drive more than 5 miles in winter when driving around town. Thus, I get a lot of error for fuel usage for city driving. But when I drive long distances on the highway where most of the time the engine is warmed up, it gives me pretty consistent fuel usage values.
But I think they used a stoichiometric value for 100 percent gasoline, not the 10 percent ethanol mix which reduces your fuel mileage, calibrating the fuel usage can correct for that.
I think it is quite useful. When one of my oxygen sensors was going bad, there were enough sensor readouts for me to figure out which sensor, etc.