I need some help deciding what gear I should get, should I get 3.73 or 3.55 gears?Right now I have a FMX transmission but I plan on upgrading to a tremec 3550 in the future. Also, I dont want to sound ignorant butI dont know the difference between open, posi-traction, and traction-loc gears so could someone give me a run down on what they all mean. Lastly, when I change the gear someone told me I have to change the bearings and seals in the rear end is this true? Thanks in advance.

- What are you building your vehicle for (street/strip/etc)?
- What are the current gears?
- What size are the rear tires you're running?

Differential gearing and traction devices are two separate things. They both end up being parts of the rear end, but they are items that you will purchase separately (and generally will get installed at the same time).

Open differential:
Delivers power to the wheel with the least amount of traction. The most classic example of how this works is if you have an "open rearend" in your vehicle, place one tire on ice and the other tire on pavement. You won't go anywhere because the tire that's on ice will get all power delivered to it. This causes traction problems and obviously if you are doing any kind of racing, you want the most traction possible.

Posi-traction:
Uses clutches to engage both rear wheels. Generally what you find from the factory in Mustangs, 4x4's, although they are offered as options. This will deliver power to both rear wheels. If one wheel is slipping, it transfers a little more power to the wheel with traction. Not as strong as a locker. Due to the nature of the clutches, limited slips will wear out with long-term use. Very streetable. These units will be in the $300+ range. Auburn is a popular manufacturer of limited slip differentials.

Lockers:
These deliver equal power to the rear wheels at all times unless you are turning sharp enough to cause enough resistance against the inside tire for the locker to disengage. The locker disengages using gearing (unlike limited slip which uses clutches). The result is a much more harsh feeling when the locker disengages. When your vehicle turns, the outside rear wheel has to spin faster than the inside rear wheel. A limited slip has a clutch pack which gently relases the traction control on the outer wheel. A locker is either engaged or disengaged, no inbetween! When it disengages, you will feel a jerk in the rearend which takes a little getting used to if you are not familiar with driving with a locked rear.

The best for traction. Most expensive, too. Detroit locker is probably the most popular. Units will run you in the $400+ range. Note that there are cheaper alternatives like a Lockright locker (can be installed in your driveway in an hour). Same concept, just not a strong as the detroit. I would recommend reading up on their web sites for detailed information on the products. Not very fun on the street. I would not recommend one in a daily driver because it's a PITA maneuvering around parking lots and in tight spaces -- especially on older vehicles without power steering.

In regards to getting new bearings and seals when you install new gears...it is considered good practice to do so. Most shops around here insist on it, especially if they are replacing the gears. If they are just adding a cheap locker, it's not really necessary. If you are removing the ring and pinion, you will probably have to pay for an overhaul kit (not too much compared to the price of gears and a traction control device).

Hope this helps...

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-Eric
SuperFord.org Admin
'67 Galaxie | '00 Excursion | Ford Excursions Registry

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Originally posted by erlong:
Hope this helps...

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Wow! How could it not help!

The only thing I might add is the powerband of the motor (i.e. cam profile, head volume, carb size, etc.) for the given application must be taken into concideration for optimal gear selection.

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Originally posted by Red_69Mach1:
Wow thanks for all that information, that helps out a lot.

I just want gears that I can use mostly on the steet, also I plan on having 350+rwhp (but thats way down the line), lastly my tires size is currently 15x7 all the way around but I plan on getting 15x8.5 in the back later on.

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I would suggest fooling around with different variables that are part of this calculation to find RPM:

http://www.superford.org/garage/calculators/index.php#finding_rpm

This will help you get an estimate as to what kind of RPM's you'll be running at at various speeds. For your reference, I have a 390 in my '67 Galaxie with a cruise-o-matic (3 speed auto) and plan on going with 3.73's and a posi rearend. It's currently running stock pizza cutter tires which I plan to replace with bigger meats in the future. This is my weekend cruiser so I am not as concerned about it being efficient and completely streetable. I think a little inconvenience (for where I'll be in the RPM range) makes it fun.

I am definitly no expert here but running a lower tire pressure will reduce the diameter of your tire. I am into four wheeling and this was a item of great debate. We did a test and the lower tire pressure definitly translates to a tire with a smaller diameter. But in the case of drag racing I would think the tire would "grow" with speed. So when going through the finish line they should be taller than their static height.

Edit: Oops... I didn't mean to bring this one back from the dead. Saw the link on another thread, saw the date, completely missed the year.
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Neal

69 Gulfstream Aqua Grande
351C/4sp
https://mustangsandmore.com/ubb/MrWesson22.html

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Alex Denysenko
Co-Administrator and Moderator

NHRA/IHRA/SRA member and licensed Superstock driver
MCA member# 53321
NHRA and IHRA SS/LA National Record Holder '00,'01,'02,&'03
Fleet of FoMoCo products including 88 ASC McLaren Mustang #28
Professional Manwhore
The Barry of BarrysGrrl

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Originally posted by TomP:
guys... unless you've swapped 8.8's or Dana's into those cars the available 8" and 9" ratios are 3.70 and 3.50

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Are you refering to what was offered by Ford? In my Summit catalog Richmond Gear offers 3.55 for the 8" and 9" rears ($189.95). No 3.73 for either, only 3.70.