What don't you like about the 4 speed Billy? I have the original 4 speed in mine and don't plan to change it. It would be nice if I could bolt on a 5th gear, but that's life.
I definitely wouldn't want to go to an automatic transmission though, as the manual is way more responsive and fun.

What is the 4 speed worth if I sell it?

Bullet Bob wrote:

Billy, the only outward advantage of the AOD over a C4 is fourth gear.  A .67 overdrive, which is slightly taller than the .68 your five speed had unless you had a Z spec five speed which has a .63 OD.  The internals in the AOD are modern and stronger than the C4 except for in 4th gear which is not strong enough to handle giant horsepower without modification.
The 1st and 2nd gear ratios in the AOD are exactly the same as the C4.
We've been running an AOD since '06...nearly 35K miles and it works fine for normal cruising.  That said, I do plan to change to a 4R70W eventually as it is just a much nicer, smoother and modern transmission which can also be fairly easily tuned as to shift points.  It's also stronger than the AOD and has higher 1st and 2nd gears which makes back shifting to 2nd or even 1st much less of a leap when mountain driving, which I do.
I think if you want to go with an automatic you will be much happier with the AOD than the C4. It will require a bit of fabricating work and custom exhaust but over all it's worth the trouble. 

BB1
 

 
Yep! All true about the AOD. My AOD was built with some upgraded internals from 4r70 and super coupe trannies.
I  have 3.25 rear gears behind my AOD and 331ci ice. My rpm is 1950@ 70mph and gets 23-24 mpg on the flat.
In retrospect the only thing I would like to have done is put a Lentec valve body in it to allow it to stay in any gear selected.

A lot of its personal preference and how the rest of the car is set up.  If you have numerically higher rear gears not having overdrive gets annoying on the highway.  I have 3.50s in my car and with the T5 it turns 2,300RPM or so at 75MPH.  I swapped from a C4 and it was turning more like 3,500RPM at the same speed.  That's a lot of RPM all the time for an engine that is done reving at 5,000RPM. 

Personally I'm of the opinion that any performance car should have a manual transmission.  I frankly don't like autos.  I've converted 3 cars from auto to manual over the years.  Sometimes its unavoidable.  My F250 would be a bear to drive as a manual.  My wife is of the same persuasion, so that's a non issue.  Her daily driver has been a manual since '00 (4 different vehicles in that time). 

If you must go auto the C4 is an inherently better design than the AOD, but you miss that 4th gear.  The AOD can be just as good, but it takes more money and parts to get it there.  You can add OD to a 3 or 4 speed using a Gear Vendors Overdrive, but they cost more than most entry level transmissions.  Actually you can split every gear with one and turn a 3 speed into a 6 speed, etc.  Neat technology, but I've only seen a handful of them in use in the real world. 

If it were me I'd go 5 speed all the way.  T5 or one of the Tremecs (TKX, 3650, etc.).  It all depends on how much engine is going in front of it. 

6sally6 wrote:

Being from Ga. you have heard the phrase........."once you go black you can never go back"??
Once you go 5 speed.........you just can't be happy with any less gears!!
You HAD a black Mustang.....You HAD a 5 speed.......of course you need a T-5..........case closed!
(Although....that 'souped-up' FMX video on here..........it sure got my attention!) WOW
6sal6

Waiting to see if my 5 speed is still good..