Differential Issue!
Thread Starter
6th Gear
Joined: Mar 2017
Posts: 6
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Differential Issue!
New to the forum first Corvette. So I got a 79 L82 4sp runs great no issues lots of power but no top end speed. Im running 4000rpms at 55mph on my speedometer and I used my gps to double check. I used some online gear calculators and the gear ratio with my 1:1 in 4th and my tire size says 6.0:1 final drive ratio? Any help would be much appreciated I'm just trying to figure out what is in the rear end so I can either have it re-geared or suggestions on where to buy a good quality rear end plus what a good ratio would be so far iv been steared twardes 3.55:1 I would like to go on the highway in the summer without blowing it the hell up. For the record I'm only driving 55mph till this is figured out!
Race Director
Joined: Nov 2006
Posts: 19,445
Likes: 353
From: Athens GA
St. Jude Donor '12-'13, '15- '16-'17-'18-'19, '21
Block front tires.
Jack up rear.
Put jack stands underneath car.
Put car into neutral.
Turn tire one revolution while counting how many revolutions the driveshaft makes.
That's your rear ratio.
So simple a caveman could do it.....
Helps if you have a friend helping with the counting.
Putting a piece of white masking tape on the driveshaft, and a black line across it, helps with seeing the # revolutions
Go from there......
Jack up rear.
Put jack stands underneath car.
Put car into neutral.
Turn tire one revolution while counting how many revolutions the driveshaft makes.
That's your rear ratio.
So simple a caveman could do it.....
Helps if you have a friend helping with the counting.
Putting a piece of white masking tape on the driveshaft, and a black line across it, helps with seeing the # revolutions
Go from there......
Last edited by bmans vette; Mar 13, 2017 at 09:39 PM.
Le Mans Master
Joined: Dec 2007
Posts: 6,422
Likes: 591
From: McHenry Illinois
First, confirm that the engine is actually running 4000 at 55 mph. Not sure there is a steep enough gear for the C3 to do that. Here is a calculator you can use to see what you have and what you want.
http://tri-5.chevyrides.com/rpm.php
http://tri-5.chevyrides.com/rpm.php
Thread Starter
6th Gear
Joined: Mar 2017
Posts: 6
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Stupid question I jacked up just the passenger side this evening and did that, one full tire spin was like 5/8 turn on the driveshaft im assuming I need to jack up both tires. And im pretty sure thats the correct rpms it runs like hell I can cruise at 35mph in 4th gear around 2400rpms no problem.
Melting Slicks
Joined: Dec 2009
Posts: 3,224
Likes: 652
From: Thunder Bay
You really need to check your tach. I doubt you are really running those RPMs. That would require a 5:1 rear ratio and I don't think its possible to install a gear that deep in the Corvette diff.
Race Director
Joined: Nov 2006
Posts: 19,445
Likes: 353
From: Athens GA
St. Jude Donor '12-'13, '15- '16-'17-'18-'19, '21
Stupid question I jacked up just the passenger side this evening and did that, one full tire spin was like 5/8 turn on the driveshaft im assuming I need to jack up both tires. And im pretty sure thats the correct rpms it runs like hell I can cruise at 35mph in 4th gear around 2400rpms no problem.
And computing your rear gear ratio by RPMs is not reliable. Too many variables like bad tachometer, poor sensor or slipping gear, cable, etc.
The only true measurement outside of removing the rear cover and checking the ratio stamped on the ring gear is to turn your wheel and observe the number of turns the driveshaft makes. That is the only solid physical connection between the 2 moving parts...the ring gear and the pinion shaft.
Good luck figuring it out otherwise.
Bman
Race Director
Joined: Mar 2016
Posts: 14,305
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From: Napa Valley California
I did this test with a GMC 4X4 a couple of years ago and it was very accurate. Just remember there isn't much difference in the rotating measurements between 3:08, 3:36 and 3:55 gear ratios so tape the inside of the wheel and a thin strip on the drive line. My '73 with a TH400 and 3:36 rear gears turns 3071 rpm at 70 mph. Does your engine sound like its actually turning 4000 rpm at 55?
Thread Starter
6th Gear
Joined: Mar 2017
Posts: 6
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Yes Iv drove it numerous times but the day I bought it I had a 3hr drive home and didn't want to go over 60mph cause I could barely hear myself think and it really felt like I was pushing it pretty hard. I'm going to Jack up "both" tires tonight and check the ratio.
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Thread Starter
6th Gear
Joined: Mar 2017
Posts: 6
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I'll check the linkage while I'm checking the rotation of the driveshaft but Iv accidentally went into 2nd from 3rd before and theres a huge difference lol, my yj 5sp had me spoiled took some getting used to shifting the 4sp with very little room between gears but I'm used to it now, much quiker and no noise going in and out makes me think about replacing my clutch in my jeep after driving the C3.
Race Director
Joined: Mar 2016
Posts: 14,305
Likes: 8,110
From: Napa Valley California
Have you done the simplest thing and call the prior owner and ask him if he knows what rear gears are in the rear end, if he changed them, if so to what ratio?
Could the clutch be slipping, or burned up?
Could the clutch be slipping, or burned up?
Thread Starter
6th Gear
Joined: Mar 2017
Posts: 6
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OK so jacked up both tires and both spun same speed got abot 3.55 rotation maybe 3.70 on the drive shaft. The tires are P255/60/R15 and on the transmission the best info I could see was 1304 065 903 - W.G.DIV. - Fc3P. The transmission is the same baby blue as the engine and looks like they have never been seperated. So thats what info I got so far and still runs 4000rpms at 55mph no issues no noises any ideas?
Thread Starter
6th Gear
Joined: Mar 2017
Posts: 6
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By the by the clutch is strong no slipping I could burn the tires 1st through 3rd no problem it has tons of power just no top end speed unless I go into the 5000rpm range and she screams bloody murder.
Melting Slicks
Joined: Dec 2009
Posts: 3,224
Likes: 652
From: Thunder Bay
OK, you know your gear ratio and you've checked your speedometer with your GPS. I have the same combination of gears and tires in my '79 and IIRC before I had an overdrive transmission I would spin about 2800 rpm at 60 mph. I think the only thing left is to verify that your tach is reading correctly.
Pro
Joined: Oct 2011
Posts: 743
Likes: 71
From: Jarrettsville Maryland
I've got a 79 with a 4 speed. When I bought it, it had a 3.36 rear in it and 80mph was right around 3500 rpms. Now that it has a 3.73 rear, 80mph is right at 3800 rpms.
I also had issues with my tach (though mine was reading way too low) that required purchasing a reproduction tach.
I also had issues with my tach (though mine was reading way too low) that required purchasing a reproduction tach.
Instructor
Joined: Sep 2015
Posts: 216
Likes: 16
From: Haverfordwest Pembrokeshire
Mine was like yours when I first got it, 4 speed Muncie and what turned out to be a 4.56:1 back end, are you sure you didn't miscount the prop turns?
I'm now running a 3.55:1 so hopefully highway running will be a little less frantic, still got a couple more jobs to do before I get it out this year so haven't driven it yet since the changes.
Graham
I'm now running a 3.55:1 so hopefully highway running will be a little less frantic, still got a couple more jobs to do before I get it out this year so haven't driven it yet since the changes.
Graham