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1977 gears changed, recalibration

LanceB

Well-known member
Joined
Jul 18, 2003
Messages
455
Location
Louisville
Corvette
1981
I had the gears changed in the rear end of my 1977, and the mechanic that worked on it never attached the speedometer cable connection under the hood. I have read that the speedometer will be off and will require recalibration, where do I put in the correct speedometer gears to make correct this and how do I know which ones to get? Also the mileage was not working either when I got the car back. If anyone has ever done anything like this your advice is much appreciated!
 
Joined
Jan 1, 2002
Messages
7,246
Location
Washington, Michigan
Corvette
'67 Marina Blue Convertible
1. What rear end ratio did you have, and what ratio did you have it changed to?

2. What transmission - 4-speed, THM350, THM400?

3. Neither the speedometer nor the odometer will work with the cable disconnected.

4. The speedometer DRIVEN gear is inside an adapter attached to the side of the transmission where the cable attaches.

:beer
 

LanceB

Well-known member
Joined
Jul 18, 2003
Messages
455
Location
Louisville
Corvette
1981
well those are good questions. The story is - it is my brothers 1977 and he met a guy at a car show that worked on sharks. This person re-did quite a bit on the car, and one of the things was new gears, I think he recommended 3.73, but they ended up going with less due to the high rpm's with that gear set. The guy working on it ended up wrecking the front end and we pulled the car. So we really don't know what gears are in it, I am not sure of the transmission, it is a 3 speed auto stock I am sure, and we are kind of starting from scratch. It runs stronger with the gears but it is unfinished with the speedometer, etc. I have purchased a lot of repair equipment since then (jacks, jackstands, etc.) and have been doing some projects on my 81, and I have been restoring some of my brothers myself, but I a little stuck on this.
 
B

bmotojoe

Guest
First connect the speedometer cable where ever it is unhooked.
Just looking at Ecklers web site there are 3 or more different ring and pinion sets that could be in there now. Finding your correct ratio without pulling the cover off and counting teeth can be done by jacking your car up and supporting it correctly with jack stands. Mark at 12 O clock (or if working on drivers side tire 3 O clock) the rear end case and driveshaft and note at 12 O clock where your tire is so you can rotate the tire completely one revolution. Turn the tire 1 revolution slowly and count how many times the mark on the driveline rotates past the mark on the rear end case. It will pass 3 times + by the sound of things. You will have to then determine what the final number is.
You will also have to know your tire size and transmission drive gear size. This is necessary in the calculation of what speedometer gear you need.
After you find what gear ratio inflate your tires to manufactures recommended pressure. I run my BFG T/A 255/60/15 5 psig higher then inside door sticker states. Take a small carpenters level and a tape measure and measure the height of the tire. Record both gear ratio and tire height.
Here are some ring and pinion sets offered by Ecklers in ratios.
3.36, 3.55, 3.70.
If your transmission is original, there were two different drive gears installed on the T-350 (M40) a 35 tooth or a 38 tooth. I used the calculator link below and changed the driven gear dropdown box to drive gear and entered 35 into the box. Then I calculated again and used 38 and there was a 1 + tooth change.
Calculator Link:http://www.wallaceracing.com/speedo-calculator.php
Hope this helps,
Brian
 

LanceB

Well-known member
Joined
Jul 18, 2003
Messages
455
Location
Louisville
Corvette
1981
Thanks Brian! I am going to put it on jack tonite and start trying to figure out exactly what gears we have. I have some stupid questions if that is ok. I am assuming the wheel will turn with the car off? Also I assume I am looking at the drive axle and I want to see it turn 3+ times when the wheel turns once. How do you figure the rest of the ratio? Do I need to mark where the wheel stops exactly and try to mathematically figure if the axle is at 55. 70, of a turn etc.? I guess the biggest thing is to get all of my ducks in a row, figure out what I have and what I need, then worry about exactly how to install the speedometer gears. The connection that I saw that was not connected was under the hood near the master cylinder. There were two cables, one had a wire sticking out of the end of it. So I should screw these back on, drive the car and see what exactly the speedometer is doing?
 
B

bmotojoe

Guest
The speedometer cable connection you’re talking about near the master cylinder is going to the cruise control transducer. If the cable that is not hooked up is going to the transmission jack the car up and have a friend watch the cable end to see if it moves when you turn the wheel by hand. If it moves then hook that cable back up to the transducer and take it for a test drive. If speedometer works then you can proceed to figure out gearing. Mark the rear end housing and driveshaft so you can see the marks from where your working. I would use a white crayon on the top of your tire and the driveshaft and housing. If the mark goes around 3 and a little more then a quarter turns I would think that the gearing would be 3.36 and a tad more then 3 and a half times on the 3.55 and so on.
There is no way to really tell what drive gear is in your transmission without pulling it apart. After you figure out your gear ratio just use the calculator and get what you need from your local Chevrolet dealer. There just as cheep there as from any Corvette vendor. Tell them you belong to a Corvette club and they should give you a 20% or more discount. Then after installing the new speedometer gear go out for a drive and a friend in another car and clock each other. You will know if figuring out the calculations using a 35 tooth internal gear or a 38. The speedometer gear if you need to get one 1 tooth different is only about $9.00 so its not a big expense if your wrong the first time, heck, after you figure out the ratio you have a 50/50 chance you’ll get it right the first time.
Brian
 

duke94

Well-known member
Joined
Jan 30, 2007
Messages
277
Location
Ann Arbor, MI
Corvette
71' convert and 78' both Red
What gear did you install? Here's a chart of the gears available for auto trannie. I have the 4 speed cahrt too in case you need it.

speedochartV76.jpg
 

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