Identify Rear Axle Gear Ratio - Ford Mustang Forum
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post #1 of 22 (permalink) Old 08-04-2019 Thread Starter
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Identify Rear Axle Gear Ratio

Is there an easy way to determine a 5.0 Fox rear axle gear ratio? Also, how can you determine if it has limited slip? Is there a coded tag on the axle somewhere? Thanks

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post #2 of 22 (permalink) Old 08-04-2019
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If the rear differential cover has never been removed or the cover has been removed,but the owner reinstalled the tag,a tag is held to the differential cover by one of the cover bolts.The tag should have 1 number/letter/ two numbers together with no spaces. Example: 3L27 This combo stands for 3:27 w/ limited slip (trak loc).....2L73= 2:73 limited slip.The only problem with this is,if the gears were ever replaced by the previous owner with a lower gear set,the tag wont be valid.So jack the rear of the car off the ground,draw a mark on the center of the driveshaft (facing the driver side) draw a mark on the rear driver tire sidewall.Slowly rotate the wheel one complete revolution while watching the driveshaft.Count the number of revolutions the driveshaft makes during one revolution of the wheel.If the driveshaft makes 4 1/4 revolutions,that equals a 4:10 gear.If it makes 3 1/2 revolutions,that equals a 3:55 gear.If the cover is removed for any reason,you can also id the gear ratio by counting the number of teeth on the ring gear and the pinion gear & divide the ring gear number by the pinion gear number to give you your answer.So 41 ring gear teeth 15 pinion gear teeth= 2.73 gear

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Last edited by wbrockstar; 08-05-2019 at 07:45 AM.
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post #3 of 22 (permalink) Old 08-04-2019 Thread Starter
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Thanks brockstar. Great answer.
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post #4 of 22 (permalink) Old 08-04-2019
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Jack it up and spin a tire. If the limited slip is working both will spin the same way. If it is not working only one will spin and or they will spin in oposite directions.
https://m.youtube.com/watch?v=8Z4Kp3hyYlk
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post #5 of 22 (permalink) Old 08-04-2019
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Thanks brockstar. Great answer.
His method is sound ONLY if both wheels turn the same amount when the shaft is turned. They rarely do, because of uneven brake friction side to side. If not a locking diff, raise only ONE wheel off the ground, count driveshaft turns for one tire revolution, then divide the number by 2. A 4.10 will give 8+ driveshaft turns.

If a locking diff, and it's clutches are not slipping, you can't turn one wheel with the other on the ground. That's how you check the clutch breakaway torque. Have to lift both wheels off ground.

All of the above: Transmission NOT in gear or park.
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post #6 of 22 (permalink) Old 08-06-2019 Thread Starter
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If the rear differential cover has never been removed or the cover has been removed,but the owner reinstalled the tag,a tag is held to the differential cover by one of the cover bolts.The tag should have 1 number/letter/ two numbers together with no spaces. Example: 3L27 This combo stands for 3:27 w/ limited slip (trak loc).....2L73= 2:73 limited slip.The only problem with this is,if the gears were ever replaced by the previous owner with a lower gear set,the tag wont be valid.So jack the rear of the car off the ground,draw a mark on the center of the driveshaft (facing the driver side) draw a mark on the rear driver tire sidewall.Slowly rotate the wheel one complete revolution while watching the driveshaft.Count the number of revolutions the driveshaft makes during one revolution of the wheel.If the driveshaft makes 4 1/4 revolutions,that equals a 4:10 gear.If it makes 3 1/2 revolutions,that equals a 3:55 gear.If the cover is removed for any reason,you can also id the gear ratio by counting the number of teeth on the ring gear and the pinion gear & divide the ring gear number by the pinion gear number to give you your answer.So 41 ring gear teeth 15 pinion gear teeth= 2.73 gear
So I just got back from a wrecking yard excursion. I found a 1988 5.0 Mustang that had a manual trans in it. Since I was looking for a manual rear axle I crawled under the car and shazam! The diff still had the factory tag on it but was so grungy it was unreadable. So I removed the tag and brought it home to clean. It has a confusing set of numbers and letters on it. Here they are:

First line: 201A
Second line: S2L73A888E16

Would the 2L73 section mean a 2:73 ratio? That seems awfully tall for a manual trans. What do you think?
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post #7 of 22 (permalink) Old 08-06-2019
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Originally Posted by rehoward View Post
So I just got back from a wrecking yard excursion. I found a 1988 5.0 Mustang that had a manual <acronym title="transmission"><acronym title="transmission">trans</acronym></acronym> in it. Since I was looking for a manual rear axle I crawled under the car and shazam! The diff still had the factory tag on it but was so grungy it was unreadable. So I removed the tag and brought it home to clean. It has a confusing set of numbers and letters on it. Here they are:

First line: 201A
Second line: S2L73A888E16

Would the 2L73 section mean a 2:73 ratio? That seems awfully tall for a manual <acronym title="transmission"><acronym title="transmission">trans</acronym></acronym>. What do you think?
My '95 is a T5 and it came with 2.73's. It'd do alomst 70 mph in second... Top speed was achieved in 4th .
2.73's are fairly common I do believe.
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post #8 of 22 (permalink) Old 08-07-2019 Thread Starter
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My '95 is a T5 and it came with 2.73's. It'd do almost 70 mph in second... Top speed was achieved in 4th .
2.73's are fairly common I do believe.
So did you change to a lower ratio? If so what did you go with and are you satisfied with it?
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post #9 of 22 (permalink) Old 08-07-2019
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So did you change to a lower ratio? If so what did you go with and are you satisfied with it?
I went to 3.37's for a good while and a year or so ago I had them replaced with 3.31. I enjoy the 3.31's more. It was kind of like cruising around in a tractor with the 3.73 and it's not significantly slower with the 3.31. I was around stock power levels when I went to the 3.73's. If I couldn't out run them with the 2.73's I still wasn't able to with the 3.73's, it just redlined quicker. It'd have to shift to 3rd to get to 60 mph and 110 or so is all it would do in 4th.
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Yeah Reo 2L73 is indeed a 2:73 rear gear.Most of the manuals came with this gear.AOD's got 3:08 sometimes I believe.Yes its a tall gear.Im surprised you found a 5.0 Mustang at the junkyard.Ive never seen one at the junkyards here,of course I primarily go to Pull-A-Part.Most of the time here,a Mustang shop called Delk Performance and a couple others seem to always get first pick on the wrecked/salvaged 5.0 Mustangs.... Even the 94-95 are only available in the 3.8 models at the junkyards,no 5.0's The 96+ SN95's are here alot though.You could possibly make some cash selling that differential online.Theyre on Ebay for $450+ and you can get the entire differential for $80 at PAP,if you can get a friend or two to ride with you to carry it or they may allow their forklift driver to come pick it up and carry it to your vehicle.94-95 differentials are even more desired.If its a regular junkyard, their price probably wouldn't make it worth buying though.

91 LX 342ci (10.5:1)
TFS 190cc (stage ll)
GT40 (tubular) intake
Lunati 61011(.549"/.565")
QA1 Coilovers
Weld ProStar XP's
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I went to 3.37's for a good while and a year or so ago I had them replaced with 3.31. I enjoy the 3.31's more. It was kind of like cruising around in a tractor with the 3.73 and it's not significantly slower with the 3.31. I was around stock power levels when I went to the 3.73's. If I couldn't out run them with the 2.73's I still wasn't able to with the 3.73's, it just redlined quicker. It'd have to shift to 3rd to get to 60 mph and 110 or so is all it would do in 4th.
I used the 3:31 ratio in my '89s that I've had as a matter of fact I have a set in my garage that I didn't get to install in my last '89 GT. I also have 3 1/2" straight stainless tips that I never installed on the '89 LX. I should probably list them for sale lol


2014 GT Premium, Track Pack, Barton shifter, Borla S-type exhaust, Resonator delete, 3.73 gears, Airaid C.A.I., 93 Race Tune, JLT Oil Separator
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I used the 3:31 ratio in my '89s that I've had as a matter of fact I have a set in my garage that I didn't get to install in my last '89 GT. I also have 3 1/2" straight stainless tips that I never installed on the '89 LX. I should probably list them for sale lol
I've thought about selling the 3.73 ring & pinion but new they're only around $160 or so. I wouldn't make enough on my used set to make them worth selling.
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[QUOTE=wbrockstar;9241903]Yeah Reo 2L73 is indeed a 2:73 rear gear.Most of the manuals came with this gear.AOD's got 3:08 sometimes I believe.QUOTE]
Manuals also came with 2.73's or 3.08's from the Factory.
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Was 3.27 a factory option? Seems like it was but maybe I just dreamed that.
I wonder what was the majority produced? I’m thinking probably 2.73. The fox I had had an aod and 2.73. It seems like if one made a boat load of power the 2.73 wouldn’t be all that bad with a T5. 70 mph in second for Pete’s sake, ha. I think you’d probably have to get a custom cam to work well with the gearing though. Mine was a bucking bronco at low speed/low rpm with 2.73 and mild cam.
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Was 3.27 a factory option? Seems like it was but maybe I just dreamed that.
I wonder what was the majority produced? Im thinking probably 2.73. The fox I had had an aod and 2.73. It seems like if one made a boat load of power the 2.73 wouldnt be all that bad with a T5. 70 mph in second for Petes sake, ha. I think youd probably have to get a custom cam to work well with the gearing though. Mine was a bucking bronco at low speed/low rpm with 2.73 and mild cam.
Yes you were right.The 82-85 auto had the 3:08.The 87-93 had the 3:27.The 3:27 wouldn't be bad in a manual either.Getting a differential out of a wrecked aod 87-93 would be the ticket for a manual with a supercharger/turbo.


91 LX 342ci (10.5:1)
TFS 190cc (stage ll)
GT40 (tubular) intake
Lunati 61011(.549"/.565")
QA1 Coilovers
Weld ProStar XP's
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