I am hoping for better mileage than that. My power band is 3000 to 6500, but it's advanced 2* at the crank. 'Crower' said that would drop my power band down about 300 to 350 rpm. He suggested a 3000 stall convertor (mostly street driven), mine is a Boss Hog 2800 to 3200. It's an 11 to 1 motor with singh grooves cut in the heads (D0VE-C's). I'm running 93 Octane Sunoco fuel with no sign of detonation. Running 3.70 gears and 27.8" tall tires so I will be in my power band from 60 mph up in high gear, C6 tranny. I built the motor but will have my mechanic fine tune it. He and his wife have matching Fox body Mustangs with 557's that they race. His is supercharged .Thrush is now part of DynoMax which is a sub of Walker. There are some good chambered street mufflers out there, but a general recommendation is for straight-through. For cruise mileage, flow is not as big a concern, and what you have should be fine for that. I would expect better than 9 mpg with good fuel and spark tuning, unless you have a high-stall converter (strip setup) with cruise RPMs outside of the stall, or gearing that is outside the torque band of your cam.