Ford Mustang Forum - View Single Post - Identify Rear Axle Gear Ratio
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post #6 of (permalink) Old 08-06-2019 Thread Starter
rehoward's Avatar
Joined: Feb 2017
Location: Everett
Posts: 243
Originally Posted by wbrockstar View Post
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 ų 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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