Well since no one has done this (as far as i can find) I've decided to give the results of my experiments. (when i first joined the site i really was wondering about this issue)
i have a v6 auto with about 53k miles on it (stock)
its true that if you turn off overdrive (the right button on your shifter) your car will not be more responsive technically but at the same time it will. By turning it off, you are stopping your cars ability to shift into 5th gear.
i have found (MANY times) that when i want to accelerate, i put the pedal half way down and the car just crawls forward in fifth. i get pissed off so i floor it, then 2 seconds later the car down shifts and accelerates . if you turn off overdrive your top gear will be 4th gear, so when you go to accelerate, you will get more pull because of the smaller gear. in turn the car feels more responsive and you can avoid the annoying lag time.........
also you can manually drop the car into third, concequentialy changing the gears faster for a better response. basically it comes down to manipulating the computer to do what you want.
now i do have the 91 bama race tune on the way, so i'm sure once i get it, this annoying problem will no longer be an issue, but i thought id just share what i learned with the other guys who are new to the mustang world.
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2006 V6 Mustang Ext. Vis: Mineral Grey w/ Yellow Stripes, Tinted windows, Shorty Antenna, Brake light chrome outline, Custom installed Fog Lights (work in progress). Int. Custom Vis: Yellow/Black Carbon fiber kit, 7 color LED ambient lights, Shaker 500 with PIE module connected to a Belkin Ipod Charger cradle. Pref Mods: Bamachips 91 race+torque tune, C&L CAI, Custom dual exhaust w/ MAC stainless steel axle back mufflers, Front and rear GT sway bar, Shelby strut tower brace.
i had a similar issue with my car not wanting to shift from 2nd to 3rd when i was doing approx 30 MPH. it would hang at 2k+ and not shift. So i played with the tune a little and modified the shift points and now i am satisfied
In terms of a tune it really does change your shift points in such a very nice way. (I have a brenspeed tune, only problem i feel is going into 5th is a bit on the rough side to me)
As far as yours hanging in a certain gear at speed the best thing i can say is just driver faster or slower. I find myself driving 45~ as its just a comfortable place in my gears.
Personal opinion on OD. I turn it off when i know i'm going to be going down some hills. Helps save on brakes. I generally try to avoid down shifting my knocking myself into 3rd as i'm not 100% sure how it will fair on my transmission in the long run.
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07 Mustang GT Metallic Alloy. + other spiffy stuff.
I have both Bama and Tillman 91 race tune. The bama is very aggressive compare to Tillman but for DD Tillman is better, reason is bama tune gives you lots of power and throttle response but as of gear change, its still will drop to 5th gear around 60 km/h however with Tillman i have less trouble because with O/D on when your foot is on gas half way it will past 60 km and go as far as 80 or sometimes 90 and still you have the great throttle response compare to stock and the air/fuel is dead on which bama leaned out just a tiny bit.
Beside that i have one issue, i actually dont know if its issue or not but when im driving and i let off gas aftere couple second of not touching gas pedal rpm drops down to 500 rpm from 1500 or what ever it is and goes up again, it exactly gives you the feeling like you put the car in first gear while cruising, i dont know why is that.
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07 Mustang Pony pkg 4.0L V6
Appearance: Roush hood scoop, rocker panel, custom GT500 chin spoiler, side louvers, racing stripes, shifter bezel, door sill plates, Sequential tail lights
Performance: C&L intake, Bama/Tillman 91 Race tune, SCT X3, Mac dual exhaust kit, OEM Ford 3.73 gears/T-lock on 7.5'
What's cheaper, a set of pads, or the increased wear and tear you're placing on your transmission and engine?
Every time I hear someone say they use "engine braking" to save their brakes, I cringe.
Brakes were designed to brake your car, your piston's rings were not.
+1 on that, hit the brakes, pads are like 50 bucks a pair, trannys arent
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2007 Satin silver 4.0-"Sadie"- Dual Pypes Violators, JLT CAI and 93 bama race tune, motive 4.10 gears and FRPP T Lok BEST E.T.- 14.485 AT 94.1
1985 Regatta Blue 'vert 5.0 LX-"Sally" 87 5.0 motor, Holley 600 4bbl carb, edelbrock high rise intake, MSD ignition, 1.7 Roller Rockers, underdrive pulleys, A/C and smog delete,BBK O/R H-pipe, flowmaster American thunders. transformed from a 3.8 to a 5.0!
i have found (MANY times) that when i want to accelerate, i put the pedal half way down and the car just crawls forward in fifth. i get pissed off so i floor it, then 2 seconds later the car down shifts and accelerates
My ‘05/V6/auto behaves exactly the same way. Wonder why Ford sets their transmissions up in such a poor manner? Wife’s V6 Solara is great on down-shifts.
I noticed you live in CA, how do you plan on getting past CARB regulations with the Bama tune? They used to do roadside spot checks on cars a couple years ago around Orange here (CHP was involved, the whole works), don’t know if they’re still doing them with the state’s budget problems.
well as far as i know its not illegal, and if i get a fix it ticket, ill just put the air box and re tune the car back to stock and take it back. that would allow me to pass.
and i havent run into cops pulling me over yet, so well see
i live right next to cal state fullerton, so there are alot of cars with exhaust CAI etc and i have not once seen a cop inspecting an engine. so if worst comes to worse swap out, pass, swap back on.
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2006 V6 Mustang Ext. Vis: Mineral Grey w/ Yellow Stripes, Tinted windows, Shorty Antenna, Brake light chrome outline, Custom installed Fog Lights (work in progress). Int. Custom Vis: Yellow/Black Carbon fiber kit, 7 color LED ambient lights, Shaker 500 with PIE module connected to a Belkin Ipod Charger cradle. Pref Mods: Bamachips 91 race+torque tune, C&L CAI, Custom dual exhaust w/ MAC stainless steel axle back mufflers, Front and rear GT sway bar, Shelby strut tower brace.
corban, i'll bet if you go ahead now and get a hand held tuner like SCT, and have your cars shift limits changed, you'll be happy. It shouldnt cost that much since all you're paying for is the hand held tuner and the tuner's time. Then later when you're ready to add the CAI, you'll already have the hand held
Brief write up on overdrive ;) 
. if you turn off overdrive your top gear will be 4th gear, so when you go to accelerate, you will get more pull because of the smaller gear. in turn the car feels more responsive and you can avoid the annoying lag time......... 
(Mods in profile)
