Rearend gear question
10-06-2017, 11:56 AM
#2
Pro
Member Since: Sep 2017
Location: Modesto California
Posts: 516
Likes: 0
Received 71 Likes
on
71 Posts
Set it on safety stands, put it in NEUTRAL, crawl under it, turn your drive shaft while counting how many turns it takes to make the tire turn one revolution. It's best to put a chalk mark or a little piece of masking tape on the bottom of the tire before turning the drive shaft so you have a good reference point. If you have an automatic transmission it'll take about 3.36 turns of the drive shaft to equal one tire revolution.
10-06-2017, 12:09 PM
#3
isnt is easier to be turning the wheel 1 turn than trying to turn the driveshaft multiple turns?
10-06-2017, 04:27 PM
#6
the stamp is meaningless if someone changed the ring and pinion
10-06-2017, 06:24 PM
#7
Drifting
exactly.. a lot happens in 4 decades .. I would not trust that its the original...spin the tire, count revs..... then you know for sure
Last edited by fishslayer143; 10-06-2017 at 06:25 PM. Reason: spilled my beer
10-06-2017, 06:45 PM
#8
I was hoping someone knew how to check vin to see what came oem .Or where this stamp is located .I know previous owner never did anything to it 
Last edited by IronMaidens2.0; 10-06-2017 at 06:47 PM.
The following users liked this post:
LenWoodruff (10-07-2017)
10-07-2017, 07:24 AM
#11
Burning Brakes
10-08-2017, 02:52 PM
#13
Le Mans Master
Just an FYI on my 78 L-82 3.70 4 speed. I have verified that the speedo is spot on for mph...60 GPS is 59-60 on the speedo. However, my L-82 GM OEM tach is not.....it reads 300-400 RPM higher revs than it should...verified 2X-once on a dyno machine and the other with a handheld tach/dwell meter..both times the tach shows 300-400 RPM more than actual revs.
The official GM speed/Tach calculations from 1978 magazine tests were for the L-82 4 speed cars with 3.70 gears:
60 mph @ 2,860 revs
I would think you should be showing about 2,600 RPMS @ 60 mph with 3.55 gears and the auto if everything is correct....around 3,000 RPMs @ 70 mph?
The official GM speed/Tach calculations from 1978 magazine tests were for the L-82 4 speed cars with 3.70 gears:
60 mph @ 2,860 revs
I would think you should be showing about 2,600 RPMS @ 60 mph with 3.55 gears and the auto if everything is correct....around 3,000 RPMs @ 70 mph?
Last edited by jb78L-82; 10-08-2017 at 02:54 PM.
The following users liked this post:
LenWoodruff (10-08-2017)
10-08-2017, 03:09 PM
#14
Melting Slicks
If the speedometer has been accurate what about pulling the speedometer cable off of the transmission and checking the color of the drive gear shaft? I think you can see it in there. After comparing what is available in most vendor catalogs wouldn't that put you in the ball park?
10-08-2017, 04:57 PM
#16
Le Mans Master
Here you go for 1978^^^^^
Almost all L-82's with a 4 speed had the close ratio gears and 3.70 gears. The wide ratio 4 speed which I have never been able to document how many were made were very rare and COULD come with a 3.36 rear gear.
All L-82's and the auto had to have only the 3.55 gear....
L-48 4 speeds came only with 3.36 gears
L-48 autos could have a 3.08 or 3.55's
Last edited by jb78L-82; 10-08-2017 at 05:02 PM.
10-08-2017, 05:11 PM
#17
Le Mans Master
Member Since: Aug 2017
Location: Cool Northern Michigan
Posts: 6,879
Received 2,112 Likes
on
1,622 Posts
Please reduce the mountain (question) back to a mole hill.
Mark tire, turn tire, count driveshaft turns, divide.
Mark tire, turn tire, count driveshaft turns, divide.
10-08-2017, 05:51 PM
#18
Burning Brakes
Just an FYI on my 78 L-82 3.70 4 speed. I have verified that the speedo is spot on for mph...60 GPS is 59-60 on the speedo. However, my L-82 GM OEM tach is not.....it reads 300-400 RPM higher revs than it should...verified 2X-once on a dyno machine and the other with a handheld tach/dwell meter..both times the tach shows 300-400 RPM more than actual revs.
The official GM speed/Tach calculations from 1978 magazine tests were for the L-82 4 speed cars with 3.70 gears:
60 mph @ 2,860 revs
I would think you should be showing about 2,600 RPMS @ 60 mph with 3.55 gears and the auto if everything is correct....around 3,000 RPMs @ 70 mph?
The official GM speed/Tach calculations from 1978 magazine tests were for the L-82 4 speed cars with 3.70 gears:
60 mph @ 2,860 revs
I would think you should be showing about 2,600 RPMS @ 60 mph with 3.55 gears and the auto if everything is correct....around 3,000 RPMs @ 70 mph?
But I think all auto L82 had the 3:55 gear?
Last edited by LenWoodruff; 10-11-2017 at 07:55 AM.
10-08-2017, 05:58 PM
#19
Le Mans Master
If you are getting 3,000 RPM @ 60 mph with 3.55 gears, then yes your tach is off as well since the 3.70 geared cars were @ 2,860 RPM at 60 mph. You should be no more than 2,600 RPM with 3.55 gears..@ 60 MPH.
The following users liked this post:
LenWoodruff (10-11-2017)
