Got the 3.90s in?

rudeone707 · Apr 6, 2004

Finally got my 3.90s and ,wow, the car freakin pulls way hard now. traction is now an issue in first gear. The guy that helped me install the gears was a little skeptical about the way the teeth were shaped, but then was surprised that they went together so smoothly. I also have no gear whine. So whoever said that the 3.90s would whine was wrong. I think that its just more the installer. The only thing i have to get now is a speedo gear and a new trak lock.

What speedo gear should i go with?

Black Stampede · Apr 6, 2004

Bite the bullitt and get a speedcal. They are worth the money.

AznStanger3v · Apr 6, 2004

Black Stampede said:
Bite the bullitt and get a speedcal. They are worth the money.

technically, they dont make a speedo gear for 3.90s. They have the 23 tooth for the 4.10s and 3.73s. Id say get the 23 tooth AND bite the bullitt and get a speedcal.

rudeone707 · Apr 6, 2004

what exactly is a speedcal and where can i get one? can i install it myself?

rockin_rick · Apr 6, 2004

The correct speedo gear setup is 6 tooth in the trans and 19 on the sensor. You won't get it very close with out a speedcal or changing the output shaft gear in the trans tailhousing.

Rick

Joes95GT · Apr 6, 2004

rockin_rick said:
The correct speedo gear setup is 6 tooth in the trans and 19 on the sensor. You won't get it very close with out a speedcal or changing the output shaft gear in the trans tailhousing.

Rick

That would get close. I'm going to agree with the majority here and say that I don't believe there is a combination of drive/driven gear made for a car with a 3.90 gear. I personally would get the Speedcal if I did it all over again. It compensates for different size wheel and tire combos and can be changed for when you go to a steeper gear . The Speedcal can be purchased from Dallas Mustang. They have install instructions on the website and the install seems incredibly simple.

Joe

94GTLaserRC · Apr 6, 2004

:worship: anything > 3.73's
RC

rudeone707 · Apr 6, 2004

94GTLaserRC said:
:worship: anything > 3.73's
RC

94GTLaserRC · Apr 6, 2004

rudeone707 said:

Youre the one that put them in, not me!
RC

urban96 · Apr 6, 2004

RC what gears do you have, stock?

GTJake · Apr 6, 2004

He's got 3.73's.

Jake

mytight95 · Apr 6, 2004

dude i think you misunderstood his post, he was saying praise anything over 3.73's. I have the 4.10's, and i will tell you however, that if you have a five speed be very careful the first time you drive it itn the wet with the gears.

lol

jason

JJ95GTID · Apr 6, 2004

Brothers Performance Warehouse also sells the Speed calibrator ($84.99). It says for 99-04 Mustangs but is the same unit as DM's. I run the 23 tooth gear with 3.73s' and it is just about dead on. If I went with 3.90s' or greater gear I would have bought the Speed Cal.

rockin_rick · Apr 6, 2004

Joes95GT said:
That would get close. I'm going to agree with the majority here and say that I don't believe there is a combination of drive/driven gear made for a car with a 3.90 gear.
Joe

That's not true. The majority is wrong then. The method that most here use is Hanlon's calculator. It is not accurate. Specifically, it does not calculate revolutions per mile for the given tire size accurately. People here believe it's data, and spread that disinformation like wildfire. I'm not trying to discredit anyone, just trying to clear up this stangnet misconception.

*************

The formula to determine speedo gears is:

rear gear ratio X drive gear teeth X tire's revolutions per mile / 1000 = driven gear

This is a well known formula and is referenced in the Ford Racing 2004 catalog on page 129.

These examples assume a tire with 810 revs/mile. This is a common size for 245/45-17's. For this example, this is not important - just that the tire size stays consistent between examples.

so with 3.90's:

3.90 X 6 X 810 /1000 = 18.954

compared to stock 3.08's:

3.08 X 8 X 810 /1000 = 19.9584

The closer that you get to a round number, the more accurate the speedo will be. The farther off, the less accurate. So for 100% perfect accuracy, you'd need a 18.954 toothed gear. Obviously, gears don't come in fractional sizes. You have to use the closest round numbered one.

The calculations show that both stock gears and the 3.90's will have nearly identical accuracy. If you assume that the speedo is calibrated to the stock 3.08/8/20 setup and takes in account for the slight inaccuracy, you have to assume that the 3.90/6/19 setup will also take that slight inaccuracy in account, and be as perfect as you can get.

This is based on math, not myth.

I discussed this in greater detail here-

http://forums.stangnet.com/showthread.php?t=389857

and

http://forums.stangnet.com/showthread.php?t=392055

Rick

mytight95 · Apr 6, 2004

yeah what he said

94GTLaserRC · Apr 7, 2004

urban96 said:
RC what gears do you have, stock?

Like Jake said...it's in my sig.

BTW, I have said it before NUMEROUS times and Ill say it again.

The Single best SOTP I ever felt leaving the shop aside from the Vortech install was when I got the 3.73's.

With an AODE, the only reason I didnt do 4.10's at the time was I did a LOT of hwy driving.

ANYONE mustang-loving enough to be in this forum and NOT have gears is cukoo!

RC

Joes95GT · Apr 7, 2004

Rick, the reason I said that the 19/6 combination would get the car close, but not spot on is because I thought Ford didn't make a 19 tooth speedometer gear. Upon further research, I found out they do. You were right . And I thought I knew it all...

Joe

rudeone707 · Apr 7, 2004

mytight95 said:
dude i think you misunderstood his post, he was saying praise anything over 3.73's. I have the 4.10's, and i will tell you however, that if you have a five speed be very careful the first time you drive it itn the wet with the gears.

lol

jason

i did misunderstand his post. I figures RC ALWAYS has something sarcastic to say. My Bad

95stocker · Apr 7, 2004

I know it's not my post, but will the 23 tooth speedo gear be the closest I can get with 3.73's?

rockin_rick · Apr 7, 2004

95stocker said:
I know it's not my post, but will the 23 tooth speedo gear be the closest I can get with 3.73's?

Without changing the 8 toother in the trans, then yes.

If you go to a 7/21, that will get you closer. Or, I suggest, a 6 and a 18. That will be just as close as the 7/21, but will allow other common rear gear changes in the future (such as 3.90, 4.10) and only require changing the speedo gear to get resonable accuracy.

Rick

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