The car readout (on your Solstice GXP?) may be using the actual pulse width of the fuel injectors, so I would trust the car display. The ScanGauge allows you to tailor/calibrate the MPG to your car at fillups to reduce error.
I like to say that the ScanGauge is a Jack of all Trades and Master of None. This is not a knock on the ScanGauge. It has to work with the lowest common denominator of data inputs that are available, as provided by the OBDII protocol. For me, I use it as a "relative" barometer of MPG. It teaches you to be a better driver, which is where a huge chunk of your MPG improvements will be coming from over time.
But the best thing to do is see which one is more accurate at gas station fill-up time.
I think the SG gets more accurate as the tank gets lower. For example if you fill up after only using 1/4 of a tank then the SG might be 15-20% off. If you filled up at almost empty then it might only be 1-2% off. I don't think any gauge will ever be 100% accurate due to variances in stations and pumps. Like Carlos said, the SG is good to use to help improve driving.
The SG will say how much fuel has been used. Compare that to your actual consumption (from the pump) and set the fill up number to that. If you make a change you'll see a +/- percentage change --> this is your calibration offset from the default.
It's all in the manual
Time is the best teacher. Unfortunately it kills all its students.
ok, did all calibrations and scanguage is now reading the same as my disp;ay so thanx for all the help
here's an interesting thing though, when I do the math my dispay was wrong also...the math shows me getting 43 miles to the gallon over a 100 mile trip, the ecu showed about 39
this is because the fill up was in a fraction that the scangauge couldn't get to and I rounded up
anyway, thanx again for all the help
by the way
has anyone used the dashhawk?...it cost double but has more readouts...like fuel mixture...it can display 7 readouts at once and it logs the trip, it also has a data base to read the codes so you don't have to look them up
looks pretty good but this is obviously a better deal if you don't use the performance readouts from dashhawk