Scanguage accuracy is limited, given that it assumes a constant AFR. The gauges and access to engine data is nice, but having something that taps into the actual fuel injector pulses like an Ultragauge would be great.
The testing really depends on what kind of mods you are doing. If it is engine related, then you really need an ultragauge or similar for optimal accuracy. If it's aero and tires related, then a coastdown test could give you the data you are looking for.
EDIT: MPGuino, not ultraguage!
My rental sentra had an MPG gauge that varied from 6.8-12% optimistic from the two fillups I did with it.
