joewill

They are not that expensive, buy them both, that way if one is still a little off, you can try another, and not pay shipping again. shipping is more than the cost of the gear.

they are cheapest at national parts depot.. 6$ each

http://shop.autocraftinvestments.com...ear&carline=20

Ken79

No cruise control. Cruise was not available with manual trans cars in 79. Would have ordered it if I could. This car did not come with a transducer on the 4-speed.

Ken79

Ordered both 18T & 19T gears. Plan to install the 19T first and check out the results. Will post my findings here.

Thanks for the help!

KevinK

So, I'm gonna jump on this wagon too....since all the experts are here...

My car had probably a 3.37 rear to begin with, but it's now a 3.08...not sure when that happened, but it did. I checked it using the chalk on tire and drive shaft/spin method, counting the rotations.

My speedo is WAY off, as told to me by my friendly local law enforcement. When my speedo says 60 in 4th gear, I'm am actually doing something closer to 75-80mph. No ticket, as they were pretty understanding, and frankly, I was only keeping up with traffic...

Anyway, what would be my setting, and also, you guys talk about the speedo cable...dumb question, where is that, and how do I get to it? Is it on the tranny itself? Thanks in advance, guys!

7T1vette

Suggestion: Make your first attempt with the 18 tooth gear. That's what you need, anyway. (Your choice, of course.)

78 silver 78

First thing is check numbers on rear end to see what gears you have. 3.08 was for auto trans. If you do have 3.08, it would be really hard on your clutch. What size tires do you have ? What year 'vette ?

KevinK

It's a 76, tires are P275/40ZR18 out back.

KevinK

And the rear end is def 3.08, I entered the data into an online speed calc, and it confirmed what the officer said. That would not be if I input the wrong rear end ratio. How would it be hard on my clutch?

Ken79

+1. Start out with the 18T. The other options are lots more work. I believe the 18T is the fewest teeth you can buy. The next option includes changing the drive gear inside the trans, and in my opinion, not worth the effort.

The speedo cable is on the right (pass) side of the 4-speed trans. A 1/4-20 bolt secures a rectangular retainer that holds the cable assy in. Loosen the bolt, and swing the retainer out of the way (do not remove the bolt as then you run the risk of losing the bolt or the retainer, which is what happened to me!). The cable with the driven gear then pulls out. Replace that driven gear with the 18T version, and your MPH discrepancy will be reduced.

78 silver 78

In '76, you had 2 4-speeds. M-20 { Wide-Range manual } and M-21 { Close-Ratio }. Standard rear end was for L48 was 3.36. You could get a 3.08 rear with the M-20 , code G-95 { Highway Ratio }.

L-82, M-20 standard was 3.55 . M-21 was 3.55 but you could get code G-92 {High Altitude Ratio } w/o AC 3.70.

With your tires and if you have the 8 tooth drive gear, you would need the 19 tooth driven gear.

If you are not having any problems taking off from a dead stop,then you might have the M-20 4 speed.

69 Chevy

Funny, I was looking for a driven speedo gear at Carlisle, too. No luck either. But Zip Products had a 10% forum member discount AND free shipping during the event. So I ordered a 22 tooth green driven gear to replace the 21 black gear.

I tried to use that calculator but it just wasn't accurate enough. So I used my handheld GPS which indicated a 3 mph discrepancy. A one tooth change, from 21 to 22 teeth, at interstate speeds of 60-75mph, corrected my speedo/odo problems.

KevinK

Funny you say that, it's a dog off the line...if I want it to take off, I have to rev it a bit, drop the clutch and floor it.

78 silver 78

Thats why it's bad on clutch.Back in the day,you had to double-clutch to get the car to roll. That ment the clutch disk wore out quite fast, that is if you have the close-ratio T-10 or munice.

KevinK

It's a Super T-10.

78 silver 78

Point is, low geared trans,high geared rearend. Not a good match.

KevinK

Oh yeah, don't gotta tell me. I'm pretty easy on the clutch for normal driving, don't gotta ride it to get her to roll or anything. But if want to launch it, I gotta drop it from 2 grand rpms just to burn the tires. I had wanted to go to 3.55 but some say that would be bad once I pony up to a tko 500, because of the low first gear it has.

78 silver 78

Are you taking Light to Light, 1/4 mile or highway ? What is tko 500?

Ken79

Updated info on the problem reported in my original post -

I replaced the 20 tooth blue gear with the 19 tooth natural gear. It essentially cut my MPH error by 1/2. With the 20 tooth gear, my speedo would read 70 and my actual speed per my GPS was 75-76. With the 19 tooth natural gear, my speedo would read 70, and my actual speed was 73-74. So, the next step is to install the 18 tooth gear, and my speedo error should be corrected.

This is what 7T1vette has been recommending all along ..... !