Rear end gearing


#1

I tested the rear end on my ford. It is not limited slip, and non-posi. One spin of the tire turned the drive shaft approx 1 and a half times. what is the rear end? and is it low or high geared?


#2

You may have to try it another way. If you had both wheels off the ground, the other wheel may have turned the other way, negating the test. If the other wheel was on the ground, your axle ratio is about 3.00 to one. That would be average gearing, neither high nor low. Low would be about 3.50 or a larger number. High would be like 2.70 or a smaller figure. Some Trans-Ams were 2.07 which was about as high as production cars got.

Mark one spot on the driveshaft with masking tape to make it easier to know just how many turns it takes.

With the one rear tire off the ground, count the number of driveshaft turns it takes to make the tire turn over twice. If it takes 3 turns, then 3.00 to one is the ratio. Three and a quarter turns would be 3.25. Kind of easy. With one tire up, the free one will turn at double speed. It’s the differential that makes it do that. It’s easy to keep track of one wheel than to have people watching both of them to make sure they turn equally. Sometimes, one won’t move.

Low means low speed and high is high speed.


#3

Spinning the wheels by hand or with the engine can tell you if it is trac-loc in good shape, or OTOH, non-Trac-loc or worn-out Trac-loc. You may need new clutches.

You might have a tag on the differential that tells if it is trac-loc.The door sticker should indicate it as well. You can find the codes if do look around. What Ford is this anyway?


#4

There is usually a tag on the differential cover that lists the gear ratio…


#5

That would be cheating, and way too easy. I forgot to mention accurate.


#6

it’s my 69’ falcon. It has a different rear end in it. I do know that it had sableizer bars on the springs. it is just a plain old rear end from what he said. This car has been rodded to hell (It’s in good shape, but blowing smoke on a 5000 mile rebuild) The guy that built it up said he got it up to 137 mph, and didn’t top it out there.


#7

Oh, well that is a horse of a different color. You can tell the ratio by counting and turning as directed. The only way to tell if it has a limited-slip and what kind is to take the cover off the pumpkin and look. It could be ANYTHING. If you are not familiar with them, I fear that your only solutions are to take to a rear-end specialist or performance shop, meet some people at a local hot-rod meet and convince them to visit your garage, or take it apart and post some pictures.


#8

Look on the door jamb for the data plate. Post the axle code on it, and we can probably tell you what ratio you have.


#9

The data plate could be wrong, OP said it had a different rear end.


#10

If you want to estimate the ratio of a car differential, you have to turn one tire two revolutions while the opposite wheel is held stationary and count the rotations that the drive shaft makes.
For more accuracy, turn the tire 10 or 20 times and divide the tire revolutions into the drive shaft revolutions and then double the ratio.

Example, 18 tire revolutions = 9 ring gear revolutions due to the 2:1 reduction of the differential gearing when the other wheel is not turning. If that results in 37 drive shaft revolutions, the ratio is 37/9 or 4.1111111 .

Point of interest, car designers do their best to steer clear of integer gear ratios like 2:1, 3:1, 4:1 etc. It can result in uneven gear wear.


#11

A 3.50 to 1 is a popular ratio. That’s 20 wheel revs to 35 driveshaft revs. But B.L.E. is correct.


#12

Oops, I missed that bit. In that case the OP can simply open up the diff. More often than not the ratio will stamped on the ring gear.


#13

Wouldn’t it work to determine engine RPM at a given speed, determine driveshaft RPM from the transmission gear ratio, and then determine differential gearing from the difference between tire speed (convert MPH to ft/sec and use 2[pi]r)?

If he knows the type of transmission installed, and can determine the gear ratios therof, he ought to get a reasonably reliable estimate this way just crunching a few numbers!