This question's been asked before - my opinion is that neither are particularly accurate.
I have an "Info Center" in my truck too, and in MY case, the dashboard value seems to be the most accurate. Others have said it's the Gryphon. I also think my dash value seems to be the more "rapidly changing" one. But, it's been a long time since I looked at either one. My new CTS has an "Inst MPG" which I look at from time to time, and it gives me an idea if I'm doing well or not. To calculate my MPG average though, I use the old-fashioned miles driven over gallons to fill the tank method. (I actually do this on a spreadsheet that I've set up to track towing MPG separate from non-towing MPG). Right now, in 3800 non-towing miles, I'm getting 15.0 mpg and in 1326 towing miles I'm getting 12.7.
- Jack