End Axles and Ratios
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
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
The Gear Ratio
The gear ratio is simply the number
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.
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.
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.
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.