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Summer 2009
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Rear End Axles and Ratios
By Ron Bramlett


Rear Ends And Ratios

    We get a lot of calls from customers who want to upgrade their Mustangs' stock rear end to a Traction Lock with a different gear ratio. Almost always, the reason is to gain performance without having to do expensive engine work. Unfortunately, most people don't know how to choose a gear ratio which will give them better performance and still be acceptable the way they want to drive their car on the street.

    What we have here is several issues and I'll try to explain then all in detail, but as simply as possible. Here goes with an easy to understand explanation (I hope!) of them all.

    In "Autotalk", when someone says "I have a 9 inch 3:25 Traction Lock in my car", what they are really saying is that their car is equipped with a 9 inch rear end, that the driveline has to make 3 and 1/4 turns to turn the rear wheels over 1 time, and that both rear wheels drive the car. It sounds simple, but let's look at each part separately. Remember that this information pertains to 1965 to 1973 Mustangs and other cars of those years.


8 inch or 9 inch?

    Ford made two size ring gears and this is what is referred to when trying to figure out if you have a 8" or a 9" rear end. The housing for 8" rear ends is more rectangular while the 9" is more round. All 1964 1/2 to 1966 Mustangs with the 289 engine had the 8" rear end except the high Performance "K" equipped cars which are very rare.


Traction Lock And Open Rear Ends


    A Traction Lock rear end will make both rear tires push the car down the road while an Open rear end has only one wheel pushing the car. This is why you will see a car with an Open rear end smoking only one wheel under severe acceleration, leaving only one black mark on the pavement. The wheel on the right has lost it's traction and is spinning wildly while the left wheel is freewheeling. Also in a mud or snow condition, when one rear wheel gets stuck and spins, the other freewheels. The Open rear end does not have the ability to move the car.

    On a Traction Lock equipped car, both tires are locked together with an internal set of clutches. This means that if one tire looses traction, the other will be able to move the car forward or backward. This is why a Traction Lock equipped car will smoke both rear tires and leave two black marks instead of just one under severe acceleration. As you can see, from a performance stand point, a Traction Lock in any performance oriented Mustang is essential. It not only gives you twice the traction, it gives you a lot more control during high speed cornering.


The Gear Ratio

    The gear ratio is simply the number
of revolutions that the drive line has to make for the tires to make one complete revolution. One revolution of the rear wheels usually moves a 1965 to 1973 Mustang a little over 6 feet.

    If you have a 2:80 to 1 rear end in your car. This means that the drive line must make 2 complete revolution plus 80% of another revolution to turn the rear tires one complete revolution. So with that in mind, it's easy to see that the drive line in a car equipped with a 3:50 rear end would have to make 3 complete revolutions plus .50 (or 1/2) of another, and that the drive line in a car equipped with a 4:11 rear end would have to make 4 complete revolutions plus .11 of another, and so on.

    It is important to remember this fact. If you change from a 2:80 rear end to a 4:11 rear end, the car will go slower at the same engine speed. Since your speedometer is operated by your transmission and will be calibrated for the 2:80 rear end, it will tell you that you are going around 90 miles per hour when you are really only going 60! Remember that you will need to have your speedometer adjusted whenever you change gear ratios.

    Also, the 2:80 equipped car will have a much higher top speed than the 4:11 car but it will take a lot longer to get there. How this effects most of us in our street cars is that we want a happy medium where we have good low
end acceleration but the car is still comfortable going down the freeway at 60 to 70 mph.


What Most Mustangs Are Equipped With

    Most 1965 to 1973 Mustangs are equipped from the factory with a 2:80 to a 3:00 open rear end. Remember, I said most. Being equipped with optional engines and transmissions can change the rear end ratio. Also, remember that any gear ratio can be a Open or a Traction Lock. You will find 3:50 Opens and 3:50 Traction locks in the same type of car. What matters is weather the car, when it was new, was ordered with the optional Traction Lock rear end. It's estimated that less than 5% of all 1964 1/2 to 1973 Mustangs were. The exceptions were the performance oriented Mustangs such as the High Performance 289 "K" Mustangs, Shelby's, Mach I's, 429 CJ's, and the Boss 302's, Boss 351's and Boss 429's. Although you would think that the Traction Lock would be standard equipment on these cars, on many it wasn't. My 1966 "K-GT" Coupe came equipped from the factory with the 3:50 Open rear end, not a Traction Lock.



The Transmission Factor

    Manual transmissions, in high gear, have a 1:1 ratio. This means that with the clutch engaged, the drive line turns on full revolution for each engine rpm. Bare in mind that if your car is equipped with an automatic transmission, the revolutions per minute (rpm) of the engine will go up because the torque converter does not convert all of the engines rpm's to usable revolutions the way the clutch on a manual transmission does. This is why, in the past 10 years or so, the auto manufactures have come up with locking torque converters and automatic over drive (A.O.D.) transmissions.


What's Right For You

    Now that we understand the meaning of a 2:80 open rear end verses a 4:11 Traction Lock rear end, let's take a look at how to choose what's right for you. Since the drive line in a 2:80 equipped car has to go around fewer times than the drive line in a 4:11 equipped car, the engines rpm's will be less at 60 miles an hour. The engine's rpm's with the 2:80 rear end and a standard transmission will be in the 2,300 range while the 4:11 equipped car will be around 3,500. Remember that the additional 1,200 rpm may not seem like much, but at freeway speeds will make the cars exhaust louder, create mor vibration, and use much more gas.

    So why would anyone want a 4:11 rear end? The 2:80's are great for gas mileage, but not for performance. It takes far more effort from the engine to start a car rolling from a dead stop with the 2:80 gear ratio than it does with the 4:11. So, the 4:11's, while not the best for freeway driving, will make the car much faster off the line and through the gears, which is exactly what you want from a performance car.


A Rule Of Thumb

    A good rule of thumb for gear ratios is as follows. With a 23 to 25 inch tall tire and an automatic transmission, use a 2:80 to a 3:50 gear ratio. Anything over a 3:50 will make the car too low geared for freeway driving. Your rpm range will be approximately 2,700 with the 2:80 to 3,500 with he 3:50 at 60 mph.

    With a manual transmission and 23 to 25 inch tall tires, use a 3:50 to 4:11 ratio. Your rpm range will be approximately 2,900 with the 3:50 to 3,500 with the 4:11 at 60 mph.


Where to Find Them

    Mustangs Plus stocks many 8" and 9" Traction Lock rear ends in a variety of gear ratios. If you want to cruise the wrecking yards for one, check out trucks, vans, and station wagons as these were more commonly equipped with lower ratios and Traction Lock units.