I have had a scangauge for a little over a year now and love it.
info can be found here
www.scangauge.com
as far as the instant MPG jumping around, the scangauge has 3 refresh settings for how fast the number refreshes. mine does change or vary greatly but the number stays there for about a half a second or better.
you can also set up an average function which shows you the average for a given time period. I have mind set up for a trip which resets after the car has been off for 3 minutes or so. this gives you the average fuel economy for a trip to town for example.
it will also tell you IAT (incoming air temp) sensor readings, O2 sensor readings, throttle position readings, I could go on and on.
there are even more advanced features like showing horsepower.
one really important feature that the scangauge has is a calibration procedure to dial in the MPG. this takes even more of the uncertainty out of the reading.
as you can see, I really like the scangauge. if you are on the fence about whether it is worth getting it or not based on quality, I can assure you the quality is above and beyond anything I have heard about the kiwi.
GasSavers_maximilian 
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