11-12 MPG seem right??

Gears don't slip, not sure what they meant by that.

I got 19mpg out of my Cobra with mix driving, and I wasn't nice to the accelerator pedal during that tank. You will probably see slightly less b/c of the converter and cams. However I think a 7mpg difference is a little much.

Seems a little low at first glance. That's about what I get in my car (see sig) in spirited mostly-city driving with the odd blast on the parkway. On the highway I've pulled down 24MPG with it (cruise on at ~65MPH).

Any codes in the PCM for O2 sensor function? Do you get the sense there's a rolling resistance problem (dragging brakes?) Is the EGR system functioning properly? What tune is in the car?

I consistantly get over 20 mpg combined, with about 24/25 when it is all highway. I have stock gears, though. But even with your 4.1's, that sounds real low ...

Is there a ton of brake dust on the rear wheels? It seems as though it is a little too too, even if some heavy spirited driving is involved. Converter and cams will lower that slightly, but it could be other things as well. My vote is for dragging brakes too!

wow. i see that mileage in a truck but even in a heavily modded stang, unless a track car wouldnd see worse then 15mpg. is it tuned? could be a problem there. how are you figuring your mileage? i get 19-22 in my 97f150, 4wd, and its 355 geared, auto, mixed driving etc. im not saying your doing it wrong but some people have misconseption of how to figure out mileage. i fill up record how much fuel is placed, then drive it, fill up again, take the mikeage ive gone and divide it by the 2nd fill up amount and comes my answer. my 97gt, see my sig got average 19-21mpg and also was not polute to the throttle, when my wife drove it she got 25-28mpg, its not for me to say but i beleive something may be wrong there, or if everything checks out then it could be a wrong combo setup you have.