Determining Gear Ratio

BallzF250

Well-Known Member
Joined
Feb 24, 2010
Location
Wilmington
I know the trick about raising the rear end, marking the tire and drive shaft, and spinning the tire 10 times while counting the drive shaft, etc etc.

Do this method work on the front drive shaft as well?

I've got an '83 Bronco that has different gear ratios in the front and rear differentials. I guess the previous owner didn't use the 4WD at all and changed the rear ratio to get better gas mileage. I'm trying to figure out what the front is, so I can match the rear to it.

The front is a Dana 44 IFS (the twin traction beam) and the rear is a 9".

Thanks in advance!
 
Pull the cover off and look... It'll give you an excuse to change the gear oil lol
 
Since it's a twin traction beam suspension, isn't it a bit of a pain to do anything with that differential (removing a bunch of stuff)? I didn't really look at it last night very close and I don't remember what it looks like.

If it is truly that easy, then what am I looking for once I get the cover off? Never done anyting with axles, diffs, etc before.
 
It will work for the front but I do not know if I would trust the accuracy of it to do a matching gear change even with 10 spins. For example was it a 4.11 or 4.56 gear ratio? That could be hard to tell by just spinning the the wheels and could be an expensive mistake on the trail. I'd pull the cover. Count the teeth on the ring and pinion then divide.
 
Wow...after looking at some pics online...it looks to me like there is no cover (just like the 9"). Looks like I'd have to pull the front driveshaft, undo the bolts from the front of the beam, and the center section removes from the back of the beam. I'm really not trying to have to go through all that.

I'll take a closer look at it tonight when I get home and see what all it entails (and look closer for a gear ID plate or something on the axle.

I understand what you're saying about the accuracy. Most of the 4WD this truck is going to see is the beach and maybe some mud. It's shouldn't hurt it that much if it was off by 4 tenths should it? I'm not doubting you...I just don't know. I could understand in a rock crawling situation how it would be very important.

Thanks for the help/info thus far.
 
The front and rear can use the same method. If the front has a locker, the number of turns of the driveshaft for each turn of the wheel will give you the ratio. If its an open diff, you will need to go two turns of the wheel instead of 1. But when doing the math, the number of turns needs to be divided by 2, because that is only 10 rotations of the ring gear due to the open differential.

Example with locker: Wheel turns 10 times, driveshaft turns 41 times. 41/10=4.1, so it has 4.10 gears. If the driveshaft turns 45.6 times for 10 turns of the wheel, it has 4.56 gears, etc.


Example with open diff: Wheel turns 20 times, driveshaft turns 41 times. 20 turns divided by 2=10, so 41/10=4.1, so it has 4.10 gears. If the driveshaft turns 45.6 times for 20 turns of the wheel, it has 4.56 gears, etc.
 
Alright so I know this has been a while...been pretty crazy busy with work and haven't really had time to work on any of my projects.

But I did manage to get out there today and give this a shot. So here's the results:

REAR - Believe this is not stock
Both tires off the ground. Rotated passenger tire 10 full rotations. Driver tire did not rotate in same/opposition direction. Driveshaft fully rotated 15 times.

FRONT - Believe this is stock
Drivers tire off ground. Passenger tire on ground (no movement). Rotated driver tire 10 full rotations. Driveshaft fully rotated 18.75 times.

Now these figures seem a little odd to me. I did manage to get smart after the fact (been on call with work all week so I have no alcohol in my system to help me think HAHA)...and look at the door label and saw H9 as the gear code...which I believe is the code for Limited Slip 3.55.

I'm almost considering just tearing into both the front and rear and replacing them both...almost...haven't completely decided on this yet.
 
What you have to do is realize that if you spin just one wheel with the other held steady it actually pushes the carrier in the diff at half the number of rotations, so divide your driveshaft rotations by 5 instead of 10, by your count:

Rear is ~3:1
Front is ~3.75:1

However a lot of things I see online show H9 for your year to be 3.55:1, so your counts may be off or the front may not be stock either.
 
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