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Can rearend gears be gauged by speedometer/tachometer?

stevolwevol

Well-known member
Joined
May 28, 2015
Messages
239
Location
Michigan
Corvette
1977 light blue
I'm trying to figure the gearing in both of my Vettes. I'm revving about 3000 at 70 mph in my 77, and I rev 3400 at 70 with my 79. So far I've read that in 79 L48 with automatic trans came with 3.55 to 1, but it seems like it has 3.70 to 1. Maybe the previous owner changed ring and pinion, or it was ordered as an option. Any input would be appreciated.
 

douglasamariani

Well-known member
Joined
May 12, 2008
Messages
170
Location
Anaheim Hills CA.
Corvette
2008 6.2 Coupe,1980 L48 Coupe,1968 427/4spd Coupe
Mark the wheel and the drive shaft with chalk or masking tape. Turn the wheel TWO revolutions and count the number of driveshaft turns. The number of driveshaft rotations will help you
determine
your rear
axle ratio
. For example, if it turned 3 and a half times, it's a 3.50:1
ratio
.
 

frank_nesta

Well-known member
Joined
Dec 24, 2004
Messages
199
Location
Los Angeles, CA
Corvette
1977 Convertible and 2017 Black Rose Grand Sport
Mark the wheel and the drive shaft with chalk or masking tape. Turn the wheel TWO revolutions and count the number of driveshaft turns. The number of driveshaft rotations will help you
determine
your rear
axle ratio
. For example, if it turned 3 and a half times, it's a 3.50:1
ratio
.

Just a little clarification. Jack the car up so both wheels will turn.

Posi: Jack both wheels off the ground. Turn one wheel 1 complete turn.
- Count the turns on the drive shaft.

Non-posi: Jack one wheel off the ground. Turn the wheel 2 complete turns.
- Count the turns on the drive shaft.

How do I know if it is a positraction rear end or not. If you turn one wheel and the opposite wheel turns in the same direction it is a posi rear end if the wheels turn opposite directions it is a non-posi reared.
 
Last edited:

frank_nesta

Well-known member
Joined
Dec 24, 2004
Messages
199
Location
Los Angeles, CA
Corvette
1977 Convertible and 2017 Black Rose Grand Sport
To answer your question. It can be done, but you need to match the year of the car, engine size, transmission and reared. Then match the RPM and speed. So it is not easy.

douglasamariani has the easiest way to figure out the rear end gear ratio.
 
Last edited:

stevolwevol

Well-known member
Joined
May 28, 2015
Messages
239
Location
Michigan
Corvette
1977 light blue
To answer your question. It can be done, but you need to match the year of the car, engine size, transmission and reared. Then match the RPM and speed. So it is not easy.

douglasamariani has the easiest way to figure out the rear end gear ratio.
Yes, It is best to count the turns by spinning the tire, although I don't have a lift, or a floor jack. Thanks, I'll figure it out.
 

frank_nesta

Well-known member
Joined
Dec 24, 2004
Messages
199
Location
Los Angeles, CA
Corvette
1977 Convertible and 2017 Black Rose Grand Sport
There is another method. See what happens when you get to sleep on something?

There is a code that is stamped into the differential case. Its located in between the leaf spring plate that is bolted to the diff. and the torque strut bracket. Its a space about 3/4" to 1" wide. You will most likely need to wire brush that area to clean it up to see what code is. If the rear end was rebuilt there is a chance the code does not apply. The best way is still counting turns of the drive shaft and wheel.

Here is the web site that has all the info you could need to figure out the code by year.

C3 Corvette Differential and Axle Codes 1977 to 1982
 
Last edited:

Tom Bryant

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Administrator
Joined
Nov 9, 2000
Messages
7,205
Location
Edgerton, Ohio, United States
Corvette
1959 black 270hp (9/2/69) 1981 Beige L81(10/20/80)
Use this chart. You will need to know the height (diameter) of your tire then check your rpm at 60 mph. Go to your tire height at the top of the chart. Then go down to the closest rpm to your car @ 60 mph. Then go to the left column and see what ratio it lists.

Chart

Tom
 

stevolwevol

Well-known member
Joined
May 28, 2015
Messages
239
Location
Michigan
Corvette
1977 light blue
Use this chart. You will need to know the height (diameter) of your tire then check your rpm at 60 mph. Go to your tire height at the top of the chart. Then go down to the closest rpm to your car @ 60 mph. Then go to the left column and see what ratio it lists.

Chart

Tom

Thank you Tom, I scoured the net for something like this. I know there are codes, and rpo's but gears can be changed. Thanks again.
 

stevolwevol

Well-known member
Joined
May 28, 2015
Messages
239
Location
Michigan
Corvette
1977 light blue
Well, at 60 I was at 3000 rpm. according to tire chart I found, with 225/70/15 tires they should be 27.4". So according to the chart you provided I have 4.11's. If that were the case, the gears were probably changed or the factory tach is less than accurate.
 

stevolwevol

Well-known member
Joined
May 28, 2015
Messages
239
Location
Michigan
Corvette
1977 light blue
Use this chart. You will need to know the height (diameter) of your tire then check your rpm at 60 mph. Go to your tire height at the top of the chart. Then go down to the closest rpm to your car @ 60 mph. Then go to the left column and see what ratio it lists.
Chart

Tom


Just a foot note to this thread, I checked my speed using GPS app, and it shows that I'm going about 3 mph faster than my speedo says. Possibly the gears were changed since L48 Vettes in 79 only offered 3.55 to 1, and the wrong driven gear was installed, or the GPS was off. Going by GPS I am reving at 2800 rpms @ 60 mph.
 

Tom Bryant

Well-known member
Administrator
Joined
Nov 9, 2000
Messages
7,205
Location
Edgerton, Ohio, United States
Corvette
1959 black 270hp (9/2/69) 1981 Beige L81(10/20/80)
I use a GPS app often and find them very accurate. A '79 with an automatic only came with a 3.55 as you know. The drive gear on the trans shaft is a code C, orange, 8 teeth, part number 6261784. The driven gear is a code P, 22 teeth, gray gear part number 3987922. It does sound like something is wrong. One other way to verify your speedometer is to go out on a highway where you can drive 60 mph and do so for one mile using the mile markers along the road for a start and finish point. It will take you exactly one minute to go one mile at 60mph. Run it again with your GPS app for comparison.

Tom
 

stevolwevol

Well-known member
Joined
May 28, 2015
Messages
239
Location
Michigan
Corvette
1977 light blue
Thanks Tom, I checked the GPS app with two of my other modern cars, and it was spot on, so I can say I have confirmed the speeedo is off in the Vette by 3 mph. This spring I will have it on a hoist and turn the tires, count the turns. Maybe even replace the drivin gear. Thanks again.
 

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