Gas mileage on 06 RT

I was wondering what gas mileage would be normal for my 06RT. This is my 2nd one (First one got totaled a few months back) and it seems like I am getting worse gas mileage. Right now, I am reading about 12.6 MPG but that is with mostly inner city driving. If I take it on the highway, for even a 20-30 min trip, it will bump up to about 13.4. I can admit that I do drive it quite hard occasionally but still, I wouldn't expect the MPG to be that low. Like this morning I started the car and it was at 12.7 By the time I sat for like 12 mins to let the engine warm up, I was down to 12.6. On my other one, even with heavy city driving, I was getting about 13.4. a few highway trips it would bump up to the mid 14's. I could be over reacting but I am just wondering if I should be concerned. There were no codes or any issues last time I was at the shop. I am not sure if it matters that I recently got a tranny pan service. I cant say it got worst since because to be honest, I didnt check before. Just wanted to know if anybody has any advice. Lastly, no real upgrades, just loudmouth resonators