Makes sense Zwede - easier to calculate that way.
Gear Ratio
Thread Starter
Melting Slicks
Joined: Sep 2011
Posts: 3,372
Likes: 215
From: Peoria Arizona
Gear Ratio
Hey all,
I am trying to figure out the rear gear ratio in our '77. I've tried the typical tire to pinion rotation method, but there is such a small difference between a 3.08 and a 3.36 (visually) using this method I just can't tell.
I am looking for educated guesses at this point. It's an L48 car with a factory (original) 4spd. Speedo isn't working yet - my main reason for wanting to know this info, so I can figure my speeds using the tach until I get the speedo working.
If I understand correctly, 3.08 was the most common gear set used? Is this true of auto and 4spd?
Thanks for any advice,
Adam
I am trying to figure out the rear gear ratio in our '77. I've tried the typical tire to pinion rotation method, but there is such a small difference between a 3.08 and a 3.36 (visually) using this method I just can't tell.
I am looking for educated guesses at this point. It's an L48 car with a factory (original) 4spd. Speedo isn't working yet - my main reason for wanting to know this info, so I can figure my speeds using the tach until I get the speedo working.
If I understand correctly, 3.08 was the most common gear set used? Is this true of auto and 4spd?
Thanks for any advice,
Adam
Race Director
Joined: Sep 1999
Posts: 11,361
Likes: 383
From: Plano TX
The GPS is the best way. But if you really want to know, just keep turning the wheel 10x. You'll be able to tell the difference between the pinion turning almost 31 times and more than 33... just don't loose count! 
Race Director
Joined: Oct 2000
Posts: 12,679
Likes: 3,130
From: Shamong, NJ
For 77, the base L-48 and 4 speed, came with the 3:36 as standard, and the 3:08 was optional. With an L-48 automatic, the only rear offered was the 3:08.
L-82 4 speed cars came with the 3:70, while the 3:55 was optional. All L-82 automatics got the 3:55.
Odds are pretty good that you have the 3:36. Less than a 1,000 of all 77's, came with an optional rear ratio.
L-82 4 speed cars came with the 3:70, while the 3:55 was optional. All L-82 automatics got the 3:55.
Odds are pretty good that you have the 3:36. Less than a 1,000 of all 77's, came with an optional rear ratio.
Last edited by gbvette62; Oct 28, 2012 at 02:36 PM.
Pro
Joined: Feb 2008
Posts: 556
Likes: 13
From: C3 Forum, Colorado
It's true that there isn't much difference between the 3.08 and the 3.36.
But using the tire to pinion rotation method shouldn't be that hard to decipher the two as long as you have a limited slip differential and it's not slipping at all during the test.
By rotating the tire one revolution it should be clear that the drive shaft will turn far less than 3 1/4 turns which you could assume it would have a 3.08 ratio.
If the drive shaft turns 3 1/4 turns and then some I think it would be safe to say it would have a 3.36 ratio.
Could it be that you don't have a good view of the drive shaft as you're rotating the tire? If so, maybe you could tape a piece of string to the bottom to the drive shaft, rotate the tire exactly one rotation and count the wraps of string on the drive shaft.
If it's a 3.08 the running end of the string should about line up with your tape mark when pulled to the center of the drive shaft where you started.
But using the tire to pinion rotation method shouldn't be that hard to decipher the two as long as you have a limited slip differential and it's not slipping at all during the test.
By rotating the tire one revolution it should be clear that the drive shaft will turn far less than 3 1/4 turns which you could assume it would have a 3.08 ratio.
If the drive shaft turns 3 1/4 turns and then some I think it would be safe to say it would have a 3.36 ratio.
Could it be that you don't have a good view of the drive shaft as you're rotating the tire? If so, maybe you could tape a piece of string to the bottom to the drive shaft, rotate the tire exactly one rotation and count the wraps of string on the drive shaft.
If it's a 3.08 the running end of the string should about line up with your tape mark when pulled to the center of the drive shaft where you started.
Thread Starter
Melting Slicks
Joined: Sep 2011
Posts: 3,372
Likes: 215
From: Peoria Arizona
For 77, the base L-48 and 4 speed, came with the 3:36 as standard, and the 3:08 was optional. With an L-48 automatic, the only rear offered was the 3:08.
L-82 4 speed cars came with the 3:70, while the 3:55 was optional. All L-82 automatics got the 3:55.
Odds are pretty good that you have the 3:36. Less than a 1,000 of all 77's, came with an optional rear ratio.
L-82 4 speed cars came with the 3:70, while the 3:55 was optional. All L-82 automatics got the 3:55.
Odds are pretty good that you have the 3:36. Less than a 1,000 of all 77's, came with an optional rear ratio.
It's true that there isn't much difference between the 3.08 and the 3.36.
But using the tire to pinion rotation method shouldn't be that hard to decipher the two as long as you have a limited slip differential and it's not slipping at all during the test.
By rotating the tire one revolution it should be clear that the drive shaft will turn far less than 3 1/4 turns which you could assume it would have a 3.08 ratio.
If the drive shaft turns 3 1/4 turns and then some I think it would be safe to say it would have a 3.36 ratio.
Could it be that you don't have a good view of the drive shaft as you're rotating the tire? If so, maybe you could tape a piece of string to the bottom to the drive shaft, rotate the tire exactly one rotation and count the wraps of string on the drive shaft.
If it's a 3.08 the running end of the string should about line up with your tape mark when pulled to the center of the drive shaft where you started.
But using the tire to pinion rotation method shouldn't be that hard to decipher the two as long as you have a limited slip differential and it's not slipping at all during the test.
By rotating the tire one revolution it should be clear that the drive shaft will turn far less than 3 1/4 turns which you could assume it would have a 3.08 ratio.
If the drive shaft turns 3 1/4 turns and then some I think it would be safe to say it would have a 3.36 ratio.
Could it be that you don't have a good view of the drive shaft as you're rotating the tire? If so, maybe you could tape a piece of string to the bottom to the drive shaft, rotate the tire exactly one rotation and count the wraps of string on the drive shaft.
If it's a 3.08 the running end of the string should about line up with your tape mark when pulled to the center of the drive shaft where you started.
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Drifting
Joined: Mar 2011
Posts: 1,347
Likes: 121
From: Kernersville NC
3.08 will not go 3.25 turns. Just over 3. If it goes a bit past 3.25 then you have a 3.36. You do need to get a good look at the u-joint for orientation and jack both sides of the rear so there is no slippage in the rear end. I would suggest getting the tires just off the ground and mark the tire and the floor with a piece of tape. Have some one turn the tire while you watch the u-joint spin. 3.55 to 3.70 is a bit harder to distinguish but yours should be relatively easy as there is more than .25 difference.
Thread Starter
Melting Slicks
Joined: Sep 2011
Posts: 3,372
Likes: 215
From: Peoria Arizona
Thanks flyeri!
I'm trying to get a idea of speed based on my tach for temporary use. I need to drive it over on Thursday to have new tires mounted and wanted to know where I'm at for that trip. Hoping to use the freeway for a short portion of it on the way home. Either way, I think I'm safe at 2,500rpm for 65mph based on a 28" tall tire.
Thanks again to all that replied,
Adam
I'm trying to get a idea of speed based on my tach for temporary use. I need to drive it over on Thursday to have new tires mounted and wanted to know where I'm at for that trip. Hoping to use the freeway for a short portion of it on the way home. Either way, I think I'm safe at 2,500rpm for 65mph based on a 28" tall tire.
Thanks again to all that replied,
Adam
Instructor
Joined: Oct 2009
Posts: 175
Likes: 1
From: Calhoun GA
Check the letters stamped on the under side of your differential, the first two letters is a code that indicates the following gear ratios:
OA ratio is 3.08
OD ratio is 3.36
LR ratio is 3.36
OB ratio is 3.55
OC ratio is 3.70
OA ratio is 3.08
OD ratio is 3.36
LR ratio is 3.36
OB ratio is 3.55
OC ratio is 3.70
Thread Starter
Melting Slicks
Joined: Sep 2011
Posts: 3,372
Likes: 215
From: Peoria Arizona
Racer
Joined: Mar 2009
Posts: 394
Likes: 1
From: Everywhere US
Adam .... I picked up a free app as well .... called Speed Tracker ... free app ...
had to use to get me started with the gear change until my new tranny speedo gear came in .....
hope that helps .....
had to use to get me started with the gear change until my new tranny speedo gear came in .....
hope that helps .....
Thread Starter
Melting Slicks
Joined: Sep 2011
Posts: 3,372
Likes: 215
From: Peoria Arizona
I'll check that out and see what's available. I use a Blackberry for my business stuff, so the apps are a little limited at times.
Thanks!
Adam