Well all things considered my stock 06 SS gets 15 mpg. You would really be splitting the difference or so. So you may get up to 12ish. Who is doing your tune? You could try leaning it out a little bit for around the town driving at part throttle if it is running any kind of rich... Also if you are running a large stall to couple with the ms3 you could probably lower your stall speed which would help your mileage for around town....