Differential gear ratio?
Thread Starter
Drifting
Joined: Apr 2008
Posts: 1,305
Likes: 2
From: Lewisburg WV
Differential gear ratio?
I'm trying to determine the gear ratio of my differential of my 1979 coupe. I have the original window sticker, but it just lists a few of the original options; L-82 engine, electric windows and doors locks, aluminum wheels, and 255/60-15 tires, etc. Since it's not listed as an option could I assume it's a standard 3:08? I have a 350 automatic. At 3500 RPM my speedometer reads 72 MPH with 255/60-15 tires. Does this sound about right for a 3:08?
Burning Brakes
Joined: Jan 2005
Posts: 847
Likes: 1
From: spring branch texas
I was to lazy to figure out your actual tire size so I guess 26" and came up with 3:08's
http://www.idavette.net/tech/ratioc.htm
http://www.idavette.net/tech/ratioc.htm
Safety Car
Joined: Oct 2006
Posts: 3,993
Likes: 9
From: Gig Harbor WA
3.70..
http://www.700r4.com/speedoCalc/rpmcalc.shtml
Note...oops sorry, 3.55, but the website is pretty slick...figures rpm vs mph for 700r4, 2004r, and 350/400...
http://www.700r4.com/speedoCalc/rpmcalc.shtml
Note...oops sorry, 3.55, but the website is pretty slick...figures rpm vs mph for 700r4, 2004r, and 350/400...
Last edited by MakoShark72; Jun 24, 2008 at 08:41 PM.
Race Director
Joined: Dec 1999
Posts: 19,610
Likes: 778
From: Forked River NJ
Your tach may be off a little also. Some of the electronic tachs had a tendancy to be off a few rpm.
Just looked up in my book and in 79 as said above the standard gear for automatic 79s is a 3.55
Just looked up in my book and in 79 as said above the standard gear for automatic 79s is a 3.55
Burning Brakes
Joined: Aug 2006
Posts: 1,155
Likes: 0
From: San Diego California
I'm trying to determine the gear ratio of my differential of my 1979 coupe. I have the original window sticker, but it just lists a few of the original options; L-82 engine, electric windows and doors locks, aluminum wheels, and 255/60-15 tires, etc. Since it's not listed as an option could I assume it's a standard 3:08? I have a 350 automatic. At 3500 RPM my speedometer reads 72 MPH with 255/60-15 tires. Does this sound about right for a 3:08?
Drifting
Joined: Sep 2006
Posts: 1,259
Likes: 2
From: MS
I think you have the stock 3.55's and it is the extra torque converter slip of the Turbo 350
that is increasing your revs further to make them seem more like 3.70's.
The 1979 axle ratio combination info straight from GM:
L48 M20 .... 3.36 all
L48 M38 .... 3.55 all
L82 M20 .... 3.70 std, 3.36 opt
L82 M21 .... 3.70 all
L82 M38 .... 3.55 all
So all automatic Vettes in 79 had the 3.55's. The 3.36 was 4-spd only and a 3.08 was used in 78 but was N/A in 79.
that is increasing your revs further to make them seem more like 3.70's.
My other info ( Corvette Chronicle by James Flammang)indicates 79 vettes came from the factory with 3.08 and 3.55 gears; and I remember one other source also indicated that 3.36 was an option that year. Unfortunately the Black Book is silent on all of this. Maybe a 79 guru out there has mo' better info to add.
L48 M20 .... 3.36 all
L48 M38 .... 3.55 all
L82 M20 .... 3.70 std, 3.36 opt
L82 M21 .... 3.70 all
L82 M38 .... 3.55 all
So all automatic Vettes in 79 had the 3.55's. The 3.36 was 4-spd only and a 3.08 was used in 78 but was N/A in 79.
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Thread Starter
Drifting
Joined: Apr 2008
Posts: 1,305
Likes: 2
From: Lewisburg WV
thegazman
Thanks for all the help. My tires are 27" diameter. Using the Idaho calculator and allowing for inaccuracies in my speedometer and tach it would seem I have a 3.70 ratio. Somewhere I read that 1979 came standard with a 3.08. The low gear ratio is great around town, but nothing to brag about on a trip, 12 miles per gallon. But even at higher speeds it accelerates without any problem. One of my goals is an overdrive transmission with a lock up converter. One of the interesting things about looking at the original window sticker was the cost of the engine package. $600 was a lot money for an option back in 1979 and the most expensive option on the car. Maybe GM went with the lower gears in 1979 to maintain acceleration while reducing the HP for smog. Thanks again for your help.
Ken
Ken
Race Director
Joined: Apr 2003
Posts: 19,350
Likes: 654
From: Tampa, Florida
Jack it up...
1. Turn wheel so valve stem is at 6 o'clock
2. Mark the driveshaft and mark the diff right next to it.
3. Turn wheel 2 or more complete revolutions.....the more turns the more accurate the results
4. count how many times driveshaft turns.
5. divide driveshaft revolutions by how many times you turned the wheel.
1. Turn wheel so valve stem is at 6 o'clock
2. Mark the driveshaft and mark the diff right next to it.
3. Turn wheel 2 or more complete revolutions.....the more turns the more accurate the results
4. count how many times driveshaft turns.
5. divide driveshaft revolutions by how many times you turned the wheel.
Drifting
Joined: Jun 2007
Posts: 1,404
Likes: 1
From: Ansonia/NYC CT
It's what I did, and I figured out I have 3.08s. Couldn't go by speed and RPM, cuz the fella that changed the rear end failed to adjust the speedo......when it says I'm doing 60, it's closer to about 85, lol.
Think a cop will buy that?
Thread Starter
Drifting
Joined: Apr 2008
Posts: 1,305
Likes: 2
From: Lewisburg WV
thegazman
Jack it up...
1. Turn wheel so valve stem is at 6 o'clock
2. Mark the driveshaft and mark the diff right next to it.
3. Turn wheel 2 or more complete revolutions.....the more turns the more accurate the results
4. count how many times driveshaft turns.
5. divide driveshaft revolutions by how many times you turned the wheel.
1. Turn wheel so valve stem is at 6 o'clock
2. Mark the driveshaft and mark the diff right next to it.
3. Turn wheel 2 or more complete revolutions.....the more turns the more accurate the results
4. count how many times driveshaft turns.
5. divide driveshaft revolutions by how many times you turned the wheel.
