Why don't you think the Scangauge works well on our cars? I use one and it seems to work in my civic.
As long as it's not the lean-burn HX it works well.
Both the SG and UG use the OBD data which doesn't report lean burn. They use airflow and assume a standard air-fuel mix to calculate fuel usage. When the mix changes, the calculated fuel is off by however big the change is. You can program the Scangauge to show when you're in lean burn. The Ultragauge can't do that.
Your best bet for accurate mpg will be the mpguino. It reads the fuel injector signal directly.