Gear Ratio Question
Thread Starter
Instructor
Joined: Jan 2008
Posts: 138
Likes: 0
From: Deland FL
Gear Ratio Question
I know this has been asked before, but I can't seem to find an answer, and I think my situation is a little different than most...How do I find out what gear ratio I have without taking the cover off and counting the teeth..On a rolling chassis..No Engine or Transmission..No Body just a Rolling Chassis..I tried pulling the chassis with another vehicle and counting the turns..and it was really high like 4.5 turns for each wheel revolution..then rolling the chassis by hand on one wheel both tires on the ground.. I got under 3 turns per tire revolution..all straight pulls no turns, at least 5 revolutions....I would jack it up and spin em but the half shafts won't turn with it jacked up...IDK..any thoughts?? thanks.J.
Racer
Joined: Dec 2007
Posts: 341
Likes: 29
From: vancouver washington
I think your on the right track. Get both rear wheels off the ground. Put a mark on the differental input yoke and rotate the wheel 1 revolution. Count the times the yoke turns. My car has 2:87 gear so the drive shaft to the trany turn almost 3 times to 1 time on the tire. 3:08 just alittle over 3 time 3:55 3 1/2 time ect.
If the wheels wont turn with the frame on jack stand try puting the stands under the rear spring at both sides or the trailing arms close to the wheels
If the wheels wont turn with the frame on jack stand try puting the stands under the rear spring at both sides or the trailing arms close to the wheels
Last edited by spikebot 81; Nov 14, 2011 at 05:14 PM.
Drifting
Joined: Dec 2004
Posts: 1,714
Likes: 42
From: Austin Texas
there is a code stamped into the housing. Its on the bottom at the very back. NOT the cover but right next to it. Its probably between the strut bracket and the spring.
find the code and go here to decipher it.
find the code and go here to decipher it.
Le Mans Master
Joined: Feb 1999
Posts: 8,351
Likes: 75
From: Tampa Bay FL
Do you know if the differential is stock to the chassis? If so, you can determine what it was equipped with from the chassis tag (ie. if it was equipped with base small block, optional differentials were, if it was L46, optional differentials were).
At minimum, with this info and the tag info you can validate if it is an original optioned differential (plus sorting out the ratio). I was surprised to find the '69 L46 with M21 4-speed offered 4.10's. But, once I researched and purchased an NCRS manual (I highly recommend for verifying what was offered stock), I learned a lot about that L46 and had lots of fun too.
At minimum, with this info and the tag info you can validate if it is an original optioned differential (plus sorting out the ratio). I was surprised to find the '69 L46 with M21 4-speed offered 4.10's. But, once I researched and purchased an NCRS manual (I highly recommend for verifying what was offered stock), I learned a lot about that L46 and had lots of fun too.
Racer
Joined: Sep 2010
Posts: 259
Likes: 1
From: Mt Airy, NC
Dont go by the code on mine, because I had the diff rebuilt from a 3.08 to a 3.55. If you know for certain no one like my self has redid anything then you can rely on the code. I would suggest look at code and then confirm.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
I don't see a problem with pushing the chassis in a straight line for one full revolution of the rear wheels and counting the revs of the driveshaft. Put an index mark at 12 o'clock on the driveshaft and count revs (to nearest 1/8th turn). That number of driveshaft turns is the gear ratio.
P.S. You can't just turn one wheel, as the unit is a 'limited slip' differential.
P.S. You can't just turn one wheel, as the unit is a 'limited slip' differential.
Thread Starter
Instructor
Joined: Jan 2008
Posts: 138
Likes: 0
From: Deland FL
well the rear code is AW which should be 3.08 gears..however it is a 1972 and all the manuals show that as not an option in 72 (every other year though)..I assume it is correct to the car..but the car is very custom..however the drivetrain appears stock..Thanks for the replies.
Safety Car
Joined: Oct 2006
Posts: 3,993
Likes: 9
From: Gig Harbor WA
well the rear code is AW which should be 3.08 gears..however it is a 1972 and all the manuals show that as not an option in 72 (every other year though)..I assume it is correct to the car..but the car is very custom..however the drivetrain appears stock..Thanks for the replies.
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Thread Starter
Instructor
Joined: Jan 2008
Posts: 138
Likes: 0
From: Deland FL
Yeah even though they aren't listed online I think they were standard on AC base engine cars..is your rear HD??.what engine do you have in your car, and what do you think of the performance with those gears??
Drifting
Joined: Oct 2007
Posts: 1,254
Likes: 5
From: Brown Summit North Carolina
I would forget the codes, on a car as old as a C3 can be, (you do not indicate the year model) there has been ample time for someone to install a new set of gears.
Counting the turns can be hard in that the difference between 3.55 turns and 3.70 turns of the drive shaft can be hard to detect, especially with the play in the meshing of the gears. With the car so disassembled, is it really that much more trouble to remove the rear cover?
Also a good time for new oil in diff.
David
Counting the turns can be hard in that the difference between 3.55 turns and 3.70 turns of the drive shaft can be hard to detect, especially with the play in the meshing of the gears. With the car so disassembled, is it really that much more trouble to remove the rear cover?
Also a good time for new oil in diff.
David
Race Director
Joined: Jul 2001
Posts: 15,892
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Standard ratio for base engine w/auto and LS5 with auto or M20. Optional with any other combo.