Yeah, that was actually the first thing I did when I hopped in the car and plugged it in. I'm thinking you're probably right about it not detecting the lean burn, or the ECU isn't reporting fuel consumption correctly when it's in lean burn mode. I went out on a reasonably empty secondary highway near my house for some test runs. I brought my GPS along with so I could confirm roadspeed with what the ScanGuage says the speed is as well as confirm slope. I ran both directions two times, and averaged the 4 runs. Total slope was 20 feet vertical over the 2 mile measured test section of road.

55mph: 42.5mpg
60mph: 39mpg
65mph: 37.5mpg

All speeds as indicated in the guage cluster. This is where it's kind of interesting; at 65mph indicated, my GPS was showing around 62.5mph and the scanguage would kind of fluctuate between 61 and 62mph. I trust my GPS more than either of those things since it's supposedly accurate to 0.2mph according to the manual. Now I know from past experience that steady cruising at 65mph or so will usually yeild more like 42-43mpg in my HX, so all I can guess is that either it's in lean burn and the ScanGuage doesn't know it, or the wheelspeed is being reported incorrectly by about 1mph or so, but even still, percentagewise, that's not enough to cover the spread from a real world average of 42 or so at 65mph indicated to it showing around 37.5.

I definitely want to do more testing; I mean the trip MPG display certainly isn't the only reason I bought it, and even if it was, I still think it's extremely valuable for comparitive purposes. Kind of like when you dyno your car, for any of you who also have cars that you tune for big horsepower; even if the dyno you use reports different numbers from a dyno your buddy uses, as long as the one you use is consistent, it's still valuable for comparitive purposes when you make changes to the car. In all my runs at each vehicle speed, it was generally almost exactly the same MPG on the display for each run, so the repeatability appears to be good.

There's probably a ton more I could say about it as I learn more, but since you're already writing a review of your own anyway, I'll just hold off and wait for you to post up whatever it is you've found so far. When do you think you'll be posting your review?