3g hemi gas mileage

I thought I would update and give some more detailed info.
I bought my gauges about 3-4 years ago and hooked them all up except the speedo so I could monitor the engine vitals as I was doing the build. These gauges are electronic and I like them alot due to the old school look of the faces. Fast forward to a year and a half ago, I get the car put back together and fire it up. All gauges work as before and from time to time I need to add gas cause I let it run in the garage and I'm checking codes and fiddling around. I finally get insurance and pay registration and take it out on its official maiden voyage to get the speedo programmed. I notice the mileage is around the 3000 mark, WTF!!! I make some calls to the tech line and the guy is really helpful and I do the speedo programming a few more times, now its damn near on the money speed wise and I happy with that, but the fuel mileage is lousy at best. After posting whats happening here, I try alot of different stuff, motor runs great, mileage sucks still. So I finally just say the hell with it and go on about getting more power. This went on since last Oct. to the end of March.

Now just last Friday, I go fill up, and reset the trip to zero, then go to Taco Bell about a half block away and sitting in the drive thru, the trip is reading 4 tenth's, WTF!!! Then I watch it go to 5, then 6, and so on. The damn thing is counting mileage while it's not moving, WTH!!! So I call the tech line this morning and the guy asks if I have all the grounds connected to one another, I say yes, he say's, seperate the speedo ground and that will do it. So I'm gonna try it and I'll post my finding's later. I sure hope this fixes the problem.