For the four speed, the 3.50 gears will be pretty steep, and nearly unbearable at highway speeds with a stock engine.  They will be great with the T5, but 3.70 would be better with a "cammed" engine.

Hope you will post a lot of progress pics and info on the Show Your Mustang Pictures and History page.

BillyC wrote:

I have a 3:55 gear in my 66 with 289 T5. It as close to perfect for the street, and an occasional run down the strip, as it gets? I had a c4 in it with the same rear gear and it was terrible. I would say if budget allows do the T5 swap when you build the rear end and go with the 3:55.

I agree !  my search goes that way ,  The only doubt is if in 5th gear the 3.55 still run on high rev on highway.
i have read that someone with same items put a 3.40 .
my goal is to have a low rev at highway speed in 5th gear.
thanks ! this tips are gold !

Michael H. wrote:

I had a powertrax locking unit and didn't care for it.  I have heard nothing but good things about the truetrack and trac-loc units.  I think 3:55 rear gears with a T5 should be pretty nice.

hi and thanks for tips .  Question : which locking unit do you have ?  lok rite , grip pro , grip lok ?
Sorry for my misundestanding but what do you mean i didnt care about it ?
 You put in place and ....nothing to say ?
good / negative impression , handling feelings , cornering , more grip ???   noisy or quiet ? 
thanks !

Alessandro wrote:

Michael H. wrote:

I had a powertrax locking unit and didn't care for it.  I have heard nothing but good things about the truetrack and trac-loc units.  I think 3:55 rear gears with a T5 should be pretty nice.

hi and thanks for tips .  Question : which locking unit do you have ?  lok rite , grip pro , grip lok ?
Sorry for my misundestanding but what do you mean i didnt care about it ?
 You put in place and ....nothing to say ?
good / negative impression , handling feelings , cornering , more grip ???   noisy or quiet ? 
thanks !

I'm not sure which version of the powertrax locker I had.  It came in a differential that I bought used.  It worked well, but the way it locked and unlocked was kind of harsh.  For casual driving it was fine, but if you went into a turn under power and then let off it would unlock in a pretty harsh manner.

The trac-loc and tru-trac are limited slip differentials instead of a locking differential so they don't cause this kind of harsh behavior.  That is according to what I have read.  I haven't actually used either of those.

Currently I don't have a locking differential or limited slip differential.  I have gone back to my 2.79 open differential... but I recently bought a 3.25 open differential I need to install.
 

Alessandro wrote:

BillyC wrote:

I have a 3:55 gear in my 66 with 289 T5. It as close to perfect for the street, and an occasional run down the strip, as it gets? I had a c4 in it with the same rear gear and it was terrible. I would say if budget allows do the T5 swap when you build the rear end and go with the 3:55.

I agree !  my search goes that way ,  The only doubt is if in 5th gear the 3.55 still run on high rev on highway.
i have read that someone with same items put a 3.40 .
my goal is to have a low rev at highway speed in 5th gear.
thanks ! this tips are gold !

 
With the 3:55 gear I run at about 1800 @ 60 mph. And get some pretty decent mileage.

TKOPerformance wrote:

Stay away from any kind of locker in a street car.  They clunk and bang something terrible making tight turns.  I ran one in my Camaro and hated it.  They also put a lot more stress on the axleshafts.  You are much better off with a good limited slip. 

Gear wise, 3.55s without OD are too steep.  The off the line acceleration will be way better than with like 2.73s, but the engine will scream at even moderate highway speeds, and a typical American V8 is not designed to turn 3,000-3,500RPM at cruise speeds; they just won't last like that.  I would do the T5 swap and the gears at the same time if at all possible, because you can properly pair the trans and rear gears for optimum all around driveability. 

FYI, I've got a T5 for sale, PM me if you're interested. 

that's why iam  here !  all you guys give me always right suggesions before throw dollars out of the window....
Also i have read about lockers noisy !  i think i will go with powertrax grip pro ( that is a lsd)..
i think i will stay between 3,00  and 3.40  in order to decrease rev at the same cruisin
when you talk about OD you mean 5 th  ?
 

The gear ratios on my AOD are: 1st, 2.40:1; 2nd, 1.47:1; 3rd, 1.00:1; OD, 0.67:1; Reverse, 2.00:1.
 Your final O/D drive ratio might be the in that range.
 When in OD my tach reads 1950 rpm at 70 mph with a 3.25 rear gear ratio. The drivetrain was initially built for turnpike driving, now that my long distance trips are tapering off I just might go to 3.55 gears for a bit more zip off the line.
 FYI I installed an Auburn gear traction control in the 8 inch rear end and it has performed admirably for many years.
 Good luck with your choices.

Last edited by Rudi (9/13/2016 3:07 PM)

TKOPerformance wrote:

I've seen a legit 1 second reduction in 1/4 mile times going from a 2.73 to a 3.55 gear in my own car.  That's the equivalent of adding 100HP to the car, because every 10HP is basically worth a 1/10th second reduction in ET.

That's awesome!  I know I'm going from 2.79 to 3.25 so it won't be as drastic, but it should still be a very noticeable difference.  I was already excited about swapping in my new chunk, but now I'm super excited.
 

The T56 in my Camaro has a 0.50:1 6th gear, with 3.73s and stock sized rear tires it turns 1,850RPM at 80MPH.  Its the interstate killer. 

A Detroit locker is different than a Powertrax in my experience. I've run both.  The Detroit tends to be smoother than the Pwertrax, given the cost difference you would expect that.  The Powertrax was designed as a cheap option for off roaders so you didn't need to mess with shims and setting backlash like when you swap a carrier.  The design is a compromise, and you can't expect the same quality from a $330 part that you can from a $550 part.

The effects of the locker are also reduced in heavier vehicles and vehicles with more torque.  I've run a Detroit in my Blazer for years and you hardly know its there.  In a car though I would still stick to a limited slip, especially a light car with a small V8. 

The trick there is that the stock Mustang T5 has a tall (numerically high) first gear of 3.35:1.  Most 5.0s were paired with 2.73 gears from the factory.  Like many things you need to look at all your gear ratios and how they affect a car's performance.  So your overall 1st gear ratio is important, because unless the engine will wind to 7,000RPM anything above about 12:1 feels very short, and you'll be shifting into 2nd almost immediately.  So, with 3.55s and a 3.35 first gear your overall first gear ratio was 11.89:1, which is about perfect for acceleration.  Guys used to run 3.73s, or even 4.10s, and I could pretty much tell what the rear gear the car had was by how they ran down the track.  With 4.10s they were shifting before the 60' mark.  It felt fast, but it actually killed their 60' time, which is the most important part of a dragrace; getting off the line.  Time saved in the 60' is basically worth double at the end of the track, so going from say a 2.00 to a 1.80 60' is going to drop your ET like 0.400, and the converse is also true. 

Why's this important?  Because if you elect to buy a Z-spec T5 it runs different gear ratios, and first is now only 2.95:1.  Ford did this because they knew these transmissions would be paired with steeper rear gears.  So with a 2.95 ratio the overall with 3.73s is now a much more manageable 11:1.  Like I said, you need to pair the trans and the rear for optimum performance. 

This is also why you avoid 4 cylinder T5s, because they had a 3.97:1 first gear to get that turd moving, but behind a V8 it acts like a granny low in a dump truck. 

Alessandro wrote:

Thanks guys you are great ! So many info for me !
Free beer in my garage !

 Castello Di Udine?

TKOPerformance wrote:

The trick there is that the stock Mustang T5 has a tall (numerically high) first gear of 3.35:1.  Most 5.0s were paired with 2.73 gears from the factory.  Like many things you need to look at all your gear ratios and how they affect a car's performance.  So your overall 1st gear ratio is important, because unless the engine will wind to 7,000RPM anything above about 12:1 feels very short, and you'll be shifting into 2nd almost immediately.  So, with 3.55s and a 3.35 first gear your overall first gear ratio was 11.89:1, which is about perfect for acceleration.  Guys used to run 3.73s, or even 4.10s, and I could pretty much tell what the rear gear the car had was by how they ran down the track.  With 4.10s they were shifting before the 60' mark.  It felt fast, but it actually killed their 60' time, which is the most important part of a dragrace; getting off the line.  Time saved in the 60' is basically worth double at the end of the track, so going from say a 2.00 to a 1.80 60' is going to drop your ET like 0.400, and the converse is also true. 

Why's this important?  Because if you elect to buy a Z-spec T5 it runs different gear ratios, and first is now only 2.95:1.  Ford did this because they knew these transmissions would be paired with steeper rear gears.  So with a 2.95 ratio the overall with 3.73s is now a much more manageable 11:1.  Like I said, you need to pair the trans and the rear for optimum performance. 

This is also why you avoid 4 cylinder T5s, because they had a 3.97:1 first gear to get that turd moving, but behind a V8 it acts like a granny low in a dump truck. 

Alessandro,  we are all different.  What TKO says is right on target if you are drag racing. 

I prefer mountain carving.  That said.  I have a T5 with the 3.35 first and a 3.0 rear end and don't like it.   Second to first downshifts have too great an rpm drop when I have to go down to first on a tight curve.  In addition, for driving around town I hate that first gear.  I want to have a longer time in first before going into second.  Shifting at around 3000 I would much prefer the 2.95 close ratio trans.  At highway speeds I'm only turning over around 1500 rpm.  The engine can handle that.  As a matter of fact it pulls from 1200 to 5500 with no problems.

I'm running a 375hp 351w In my '66.  I do not need as much rpm to do what I want considering the car weighs less than 3,000 lb (1361kg).  I seldom go over 5,000rpm.

I hope this does not confuse you too much.