what mod's have you people astuly put on your car and then tested to see how bad your milage droped. id like to know what to put on my car (to get better gas milage) mustangs are great famialy car they have a back seat they are not a sports car thats what make's it so cool to see a cobra walk all over vett's or a viper
Since no one else has bothered to help you, I will.
For each mod, you just need to consider if it makes the car/engine more efficient, or if you are adding a resistance or hurting efficiency in normal cruise rpms.
Mods that can hurt milage:
Gears, you are hurting efficiency by making the engine rev higher.
Blowers, it always takes extra power to spin a blower
Wider tires or heavier wheels, more rolling resistance and higher weight (minimal change)
Cams, generally hurt efficiency and power below 2500, where you spend most of your time in daily driving
Bigger fuel injectors, but if tuned properly these won't hurt you
Mods that don't change or even help milage:
freeing up the intake/exhaust, improves efficiency, can be measurable
turbochargers, these can definately add milage, look at previous posts in this thread

underdriver pulleys, less drag
weight reduction
porting, improves effiiency
a tune, more timing lets the engine be more efficient, ensuring the AFs are good before the O2s warm up.
You get the idea...
But you would be seriously amazed by the differnce in milage when you change your driving style. You can easily gain 3-4mpg by not riding the ass of the person in front of you, and short shifting. You can even keep your foot in it when you are short shifting. I'll often do a 1-3-5 or 2-4-5 shift pattern to keep the rpms down. Giving it more throttle from 1000-1800 rpm really is the best way to get to speed for milage.
My best milage was 27mpg, but I would get sick of driving like that and end up with 24-25. These days, I'm stuck in traffic too much, and doing to much city driving, so my milage sucks.. about 22.