65 L79 4-Spd / Speedo / Odo Gear Trouble

[Glenn Rogers]

I was hoping someone here may have some insight to several questions about my 1965 L79 4-spd and a speedometer/odo issue that no one has been able to fully answer from other forums or at local Corvette repair shops.

I apologize in advance for the length of this posting. I'm the original owner of a 1965 2-top Nassau Blue & black L79 350HP 4-Spd w/PS which has remained mostly original and is driven daily. Its build date was April 1965.

My questions are:

1. Were all 4-spd 1965 L79's supplied with the close ratio 4-spd or were the wide ratio units also available as an option?

2. Was the 1965 L79 close ratio 4-spd option referred to as an M-20 or an M-21?

3. Is the 1.77" OD blue internal drive gear GM # 6261795 unique to the close ratio transmissons and the 1.85" OD green internal drive gear GM # 6261794 unique to the wide ratio units?

4. Are the number of teeth on these two different drive gears the same?

5. What would happen if one of the incompatible driven gears are installed?

6. Were all posi axle ratios available for the 1965 close ratio L79 4-spds, or were only the 3.70 to 4:56 ratios offered?

My current tires provide 757.5 revolutions per mile which is close to the original 760 design basis. I recently had the cluster pulled at a corvette shop to get the odometer working again and a replacement full housing speedometer head and odometer driven gear were installed. The driven gear at the transmission was not altered.

Now that the odo wheels are turning, I used a portable GPS unit to check accuracy and surprisingly found that the odo is reading 20% high (1.2 miles recorded for every true mile traveled) while the replacement speedo head that the shop installed reads 20% slow (just the opposite of what one would expect).

Does this sound like I'm suffering from both a miscalibrated speedo head (reading slow) and mismatched transmission nylon gearing (cable spinning too fast)?

I had the transmission completely rebuilt 10 years ago and believe I was told it had a 2:20 ratio first gear, but I have not truly verified that statement. My ring and pinion gears at the rear axle were also replaced at that time with 3:36 gearing which made the speedometer read 10% slow. The original gearing was either 3:55 or 3:70 (do not exactly recall). I replaced the original nylon speedometer gear at the transmission with what I believe to have been a 20 tooth blue gear and for the last ten years the speedometer has been dead accurate. I unfortunately do not recall the color or tooth count of the original driven gear and did not look for the color of the internal drive gear. Incidently, the odometer wheels stopped working about 20 years ago before the rear axle ratio was changed.

The only thing I can think of is that my old speedometer head must have been miscalibrated to 1200 RPM at 60 MPH rather than 1000 RPM and so it was reading accurate in spite of an over-speeding cable (assuming that is what's causing the working odometer wheels to sprint faster). Or maybe I'm missing something else here.

Never-the-less, the shop now propsoes to charge me additional funds to hang some kind of an adjustable contraption off the side of the transmission to supposedly fix the speedometer and odometer errors (which again, are errors in opposed directions to each other).

Now, I almost wish I had left the odo wheels frozen.

Again, I apologize for this long-winded posting but anyone's expert thoughts on this would be greatly appreciated.

Thanks .... GR

[William C.]

Re: 65 L79 4-Spd / Speedo / Odo Gear Trouble

OK, the standard setup in '65 was to use a close ratio trans with a 3.70 rear end or lower (higher numericaly) and a wide box with the 3.08-3.36-3.55 rears. The large vs small drive gears have a different tooth count to facilitate the speedo calibration. (six vs seven, or seven vs eight, I forget right now. Using the large drive gear with the large driven gear will destroy the driven gear. Using the small drive gear with the small driven gear will result in no drive to the speedo. I have to run an errand with my wife now so I can't go into more detail at this time, I'm sure others will pitch in, but I hope this gives you some general direction...

[Gerard F.]

Re: 65 L79 4-Spd / Speedo / Odo Gear Trouble

Glenn,

I always thought that if your odometer was reading high then your speedometer is reading high by the same percentage, although I could be wrong. I'd like to hear from others on this. Here's a nifty website where you can calculate your speed from the rear axle ratio and tire size:

http://www.angelfire.com/fl/procrastination/rear.html

I don't think the M21(close ratio) 4 speed was available as an option in 1965, but the M20 was. I think both of the transmissions are 1:1 in 4th gear.

Hope this helps, and I'd like to find out about that difference in the odometer and speedometer myself, as I use post mile markers to calculate the correction of my speedometer. As I understand for an older CHP, that's the way they used to do it.

Jerry Fuccillo
#42179

[Joe L.]

Re: 65 L79 4-Spd / Speedo / Odo Gear Trouble

Glenn-----

1) For 1965 the L-79 was available only with a close ratio 4 speed transmission;

2) Both close and wide ratio transmissions for 1965 had an RPO designation of "M-20";

3) The 1.77" DRIVE gear is not unique to applications using close ratio transmissions. However, it is used, primarily, for applications using higher numerical rear ratios. Since cars with close ratio transmissions are, generally, the ones that use higher numerical rear drive ratios, it follows that the 1.77" DRIVE gear is GENERALLY used with cars with close ratio transmissions. But, it's certainly NOT a "hard-and-fast" rule. The 1.85" DRIVE gear is generally used for applications with lower numerical final drive ratios. These ratios are GENERALLY used for cars with wide ratio transmissions. So, it follows that the 1.85" DRIVE gear is GENERALLY used for cars with wide ratio transmissions.

Also, the GM #6261795 and 6261794 DRIVE gears are NOT APPLICABLE to a 1965 M-20. These are NYLON drive gears and require a special clip for retention to the main shaft. The 1965 mainshaft does not have provisions for the clip. 1965 used DRIVE gears manufactured of steel and a press fit on the mainshaft. In terms of OD they are the same as the 6261795 and 6261794, though. These were GM #3708144 (1.85")and GM #3708145 (1.77"). All of the DRIVE gears are GM-discontinued except the 6261794;

4) All of the DRIVE gears have 8 teeth;

5) Use of a 1.77" DRIVE gears with a 22 tooth SILVER DRIVEN gear or any lesser tooth DRIVEN gear will cause partial engagement of the gears and result in poor operation or failed gears in short order. Use of the 1.85" DRIVE gears with 22 tooth GREEN or any greater tooth count DRIVEN gear will cause rapid deterioration of the gears and, subesequently, failure;

6) For 1965 with L-79 only 3 ratios were available : 3.70:1 (standard ratio available with or without posi), 3.55:1 (economy ratio available with posi only), 4.11:1 (performance ratio available only with positraction). The 4.56:1 was not available with an L-79 for 1965.

If your car has a 3.55:1 axle ratio, I would say that you should have a GM #3708144 DRIVE gear with a 21 tooth RED DRIVEN gear of GM #3987921. If you have a 3.70:1 final drive ratio, then I would say that you should have a GM #3708145 DRIVE gear with a 22 tooth GREEN DRIVEN gear.

The axle code stamped on the bottom of your differential carrier will tell you which rear end you have:

"AN"= 3.55:1

"AS" or "AO"= 3.70:1

[Mike G.]

Re: 65 L79 4-Spd / Speedo / Odo Gear Trouble

Confirming Clup's input, my original '65 L79 with a 3.55 ring/pinion was equipped with a WIDE ratio M20 (no grooves on the input shaft).

[Glenn Rogers]

Re: 65 L79 4-Spd / Speedo / Odo Gear Trouble

Joe ... many thanks for the help and info.

Now that I have my odo wheels turning at 1.2 miles for every mile driven, doesn't that mean the speedo cable is turning too fast (about 1200 RPM at 60 MPH instead of 1000 RPM). Aren't these anolog speedos are typiclly calibrated to the speedo cable turing 1000 RPM at 60 MPH. As I recall, isn't the odo also directly driven off a #2 worm hole right off the speedo head input shaft?

[Joe L.]

Re: 65 L79 4-Spd / Speedo / Odo Gear Trouble

Mike-----

That may be but it's contrary to what GM said was ever made. I cross-checked this from 2 GM sources since I thought that the 65 L-79 was available with either wide or close ratio 4 speed. Both indicated that 1965 L-79 was available only with close ratio transmission.

A few questions: do you know the gearset in the transmission to be original to the case? Do you know the transmission to be a wide ratio from some other means than the fact that it has no grooves on the input shaft? Many SERVICE and aftermarket replacement shafts did not have any grooves on the input shaft regardless of which transmission it was for.

[Joe L.]

Addendum

Also, for 1963-65 all 4 speeds were designated M-20. However, there were, of course, the 2 first gear ratios for the M-20. The ratio that one got was determined by the engine option and, otherwise, there was no means of specifying the transmission first gear ratio. If one wanted a 4 speed, one ordered M-20; that's all one could do. The first gear ratio was "decided" by the engine option. 250 hp and 300 hp got the 2.56:1 first gear ratio M-20; 350, 365, 375, and 425 got the 2.20:1 M-20.

For 1966, things changed. Now available were M-20, M-21, and M-22. For the base engine, only the wide ratio M-20 (or, Powerglide) was available. For L-79 and L-36, one could specify either the M-20 or M-21. For the L-72, only the M-21 (or, M-22) could be specified.

[Gerard F.]

Re: 65 L79 4-Spd / Speedo / Odo Gear Trouble

Glenn,

In reviewing your original post, I think the problem is more with your replacement speedometer head and the replacement odometer gear then the gears in your transmission. Something is not right between them.

If the speedometer was once "dead on" and then with replacement of the head and odometer gear, the speedometer is 20% slow and the odometer 20% fast, there is something wrong between the two. Changing the speed of the cable is not going to change this mismatch.

As I understand from an article in the Restorer (Winter 86), there is no direct connection between the cable and speedometer needle. The needle is turned by a magnet on the cable. The odometer is turned by a gear on the magnet shaft and a second gear on the odometer. Maybe there is a mismatched gear between gear on the magnet shaft and the odometer gear. The article says that the second gear, if replaced should be with a plastic gear so that it doesn't bung up the gear on the magnet shaft.

Both the odometer and speedometer should be in synch if the speedometer magnets are calibrated correctly. If the odometer is 10% high then the speedometer should be 10% high. Then you can correct the problem with the cable speed and gearing at the transmission.

Considering that you once had the speedometer dead on, I think you may have the wrong speedometer head or the wrong odometer gear.

Just my two cents.

Jerry Fuccillo
#42179

[Glenn Rogers]

Re: 65 L79 4-Spd / Speedo / Odo Gear Trouble

Thanks to everyone for the help ..... I'm going to have the cluster pulled again by the shop who did the work and bring in a known correct head properly calibrated and a matching odo driven gear set and watch them put it in this time. If that solves the problem, I'm going to insist they refund me the $220 they charged me for the parts they originally installed by taking them back.

Glenn

[Gerard F.]

Re: 65 L79 4-Spd / Speedo / Odo Gear Trouble

Glenn,

Here's something I got off the net under speedometer calibration, for what it's worth:

"One important piece of information which is required is the number of revolutions it takes your speedometer to register one mile. On most modern cars, this may be found on the bottom (or elsewhere) of the speedometer face, usually in very small numbers. Look for something like 940, 960, 1000, or 1020, or something close. This number is the number of speedometer cable revolutions (or equivalent in the case of electronic sender units) to increment the odometer one mile. Most speedometers (modern ones) are 1000 revs/mile, and this is a good starting number to use."

Take a look at your original speedo and see if you can find the above number. Then make sure they put a head in with the same spec number. I'll bet you will find that the number of the one they put in is 20% off the original.

Let us know if this is the resolution of the problem.

Jerry Fuccillo
#42179

[Glenn Rogers]

Re: 65 L79 4-Spd / Speedo / Odo Gear Trouble

Thanks Jerry ... I looked on the face of my 65 speedometer plate and found nothing but it may be on the back of the face plate which I can't get to.

I thought all 63-67 Corvette speedometers were calibrated to 1000 RPM for 60 MPH.

Maybe someone else can confirm that?

I also found no numbers on my original head that is pictured below:

[Gerard F.]

Duke Williams- Where are you on this one?

Glenn,

I think a calibration of 1000 RPM @ 60 MPH for the speedometer is 1000 revs per mile for the odometer, or should be. 1000 rpm X 60 min/hr divided by 60 mph = 1000 revs per mile. I might be all wet on this one, but maybe Duke Williams can dry me off and straighten me out.

Maybe the 63-67 Corvette speedometers had a different revs per mile factor?

Jerry Fuccillo