I've owned a SGII for about 9 months now. I have yet to get the calibration correct.
The best I can describe it is that on Low Fuel Flow rates it reports Low, and on High Fuel flow rates it reports High.
So if I drive it on the highway with my "in town" calibration my SGII indicates very high consumption (around 30mpg). If I drive all Highway with the cruise (75~77 mph) my fillups are in the very low 50's. (non Hypermiling driving)
If I Drive in the City with my "Highway" calibration I can get my indicated adverage mileage to go very high. Normal (non Hypermiling) in the mid 80's (mid 40's based on fillups) and Hypermiling in the 300 MPG indicated range (actual based on fillups Low 70's)
What I've Done
Tried 3 different SGII units
Replaced Thermostat (appears to have improved situation, but still unsure)
Tried Driving "normal" God I hate doing that now.
Tried filling up around half tank to increase the number of Data points.
Trying the 1/2 correction now, but I think all I'm getting is a better average, still off in same direction for highway/ City driving.
I'd like to say that I still love the SGII, and I do not believe the issue to be with the unit, but with my Golf in some way. All of the other ones I'm around work GREAT.
Theories
1)Partially Plugged Injectors flow well at low pressure, and poorly at high pressure (incomparisn to new)
2) Injector "lift" sensor faulty/ in accurate
3) ECU issue
4) Something I can't think of.
Here's a thread I posted in on TDI Club.
http://forums.tdiclub.com/showthread.php?t=203066
As a side note I have been PM'd by 4 other cars with the same issue who were afraid of posting because I was ostracized for posting in another thread which has since been deleted.
Also I always fill my car to the Brim (normal practice on a Diesel) so it is safe to say that my fillups are with in 1/4 Gallon and should be much closer than that.
Any and All Ideas Welcomed