Daily driver...and occasional bash trip 

Well I guess it depends on intended use. I am ASSuming a daily driver, and that generally means freeway RPMs as one of the main concerns. But also what's the trans gearing? And yeah like you mentioned RPM operation of the motor (cam shaft, cylinder heads, intake, etc). Rear tire size also matters. Personally I like a steeper gearing, my Lightning still has its factory 4:10's, but it also has 30" tall rear tires and an overdrive. 

Not sure its worth the expense to change from a 3.25.  With a single overdrive like the T5 that's something like a 0.68:1 and a reasonable tire in the 25-26" height range you'll have good cruising RPM and 1st gear won't be like a granny low in a dump truck.  With a double overdrive like a T56 where 6th is 0.50:1 you can run a 4:10 no problem.  I have a 3.73 rear in my IROC with a T56 and at 80MPH it turns 1,800RPM.  A T56 likely doesn't fit in an early Mustang though. 

I am planning a 3.25.1 so far

Coupla thoughts.......
Snotty idling cams really need stiffer gears so the engine can 'get-up-on-the-cam' (smooth out and get into its power range) by the time you're in 5th gear.
Uhhhhhh...mine still has a little "stutter" in 5th gear at 55/60 mph !
I don't kear.....I think it's sorta cool really.
Butt ideally speaking.... engine should be running smooth and starting to pull strong(ly) when going into OD.
Also.....take into consideration the weight of the vehicle...tire size(diameter)...freeway vs skreet use.
If it-ain't-heavy......he's my brutha..
the engine can pull a light car....like a 65 Mustang along pretty good even when its stuttering and loping a little.
Big 'unz not-so-much.
SOOOOOOOO...3.25:1 will be perfect for your car.  Install & Enjoy !
6sal6

TKOPerformance wrote:

Not sure its worth the expense to change from a 3.25.  With a single overdrive like the T5 that's something like a 0.68:1 and a reasonable tire in the 25-26" height range you'll have good cruising RPM and 1st gear won't be like a granny low in a dump truck.  With a double overdrive like a T56 where 6th is 0.50:1 you can run a 4:10 no problem.  I have a 3.73 rear in my IROC with a T56 and at 80MPH it turns 1,800RPM.  A T56 likely doesn't fit in an early Mustang though. 

Tire is 225/55/16    and 25.7 tall

BILLY WALTON from GEORGIA wrote:

Steve69 wrote:

I really like my 3.25.  

Are you runnig a 5 speed and what is your tire height?
 

   Im running a T5.   275 40 17 in the rear and 235 45 17 on the front.  


 

Did not see what transmission you intend to install. I have a T5, 2.93 first gear, .63 5th gear, 3.55 differential ratio, and 13 inch dynamic tire radius powered by a 347. The T5 offers different first and fifth gear ratios, and I chose the transmission, differential ratio, and tire size to work for my car and am very satisfied with the performance and driving experience. if you get close to an optimum combination of tire/trans/diff ratio, you will be fine. I would also calculate driveshaft rpm, although it is a non-issue with most tire/transmission/differential combinations. Good luck.

Think you’re gonna need more pizza and beer.😁

I have a stock 5.0L/stock T5/3.25 w/LSD.

I had initially installed the 3.25’s with the 289/C4 and it worked great.

Fifth is truly an overdrive with my current combination.

Tire width for traction is limited on the 65/66…without modifications of course.

That’s a distant future project…maybe.

My intended use is cruising/daily driver during the non snow covered road season.

I think summer was on a Tuesday last year.

Plenty of gear, not enough tire.

BobE wrote:

I’d run the 3.25s and see how it works.  With your T5, (3.35 first gear) I believe the 5th gear OD is a 0.68 ratio which would bring down the rear end ratio to 2.21 (when is fifth gear).  With 25.7” high tires you likely will be at about 70MPH at 2000RPM.  Then the next step is the engine is ‘happy’ running at this RPM, and that depends mostly on the cam you are running. 

I currently have a 3.00.1 open rear i it now.
 

This cam is very similar to the cam I’m running, although mine is a roller cam (Comp Cams), I have 1.6 rocker arms.  The listed RPM range for my cam is 1800-5800.  I suggest verifying with your cam manufacturer the RPM range for your cam.
 
Some data for you to consider.
With a 3.0 rear and a 0.68 5th gear, the rear end ratio would effectively be 2.04. 
Using 2.04 as the rear gear:
In 5th gear, at 60MPH, the RPM would be about 1600.
In 5th gear, at 70MPH, the RPM would be about 1900
In 5th gear, at 80MPH, the RPM would be about 2100
 
With a 3.25 rear and a 0.68 5th gear, the rear end ratio would effectively be 2.21. 
Using 2.21 as the rear gear:
In 5th gear, at 60MPH, the RPM would be about 1700.
In 5th gear, at 70MPH, the RPM would be about 2000
In 5th gear, at 80MPH, the RPM would be about 2300
 
With a 3.55 rear and a 0.68 5th gear, the rear end ratio would effectively be 2.41. 
Using 2.41 as the rear gear:
In 5th gear, at 60MPH, the RPM would be about 1900.
In 5th gear, at 70MPH, the RPM would be about 2200
In 5th gear, at 80MPH, the RPM would be about 2500
 
 
My 65 w/tire size similar to yours, 3.89 rear, in 5th gear my rear end ratio would drop to 2.45.
In 5th gear, at 60MPH, the RPM would be about 1900.
In 5th gear, at 70MPH, the RPM would be about 2250
In 5th gear, at 80MPH, the RPM would be about 2600
While I’m comfortable with this setup, I’m may go to 3.70 rear that would reduce the above RPM numbers by about 200-250.
 
Hope this helps.