I've had my 63 for about 4 yrs. From day-1 I knew that the speedometer was not accurate. From driving on the highway, I guesstameted that when it was reading 50 mph that I was probably doing about 70 mph. Don't know why it took so long, but last night I went for a ride and took a GPS with me. Readings were weird. 20 was 30. 30 was 40, but 40 was 55 and 50 was 70- 55 was 80. I'm also wondering if my tach is accurate. When the speedo is reading 50 (70), I'm taching 2500. Is there an easy was to correct this issue? I think that a prior owner, changed the rear end ratio. Is this something that I could do myself??

 

The first thing you have to do is the determine the rear-end gear ratio. Likely the rear-end was changed from a lower ratio to a higher ratio for highway cruising. Either that or the rear exploded and this was the only rear gears the previous owner could find. Once you find out what the rear ratio is, it is a matter of finding the correct gear for the transmission to be compatible with the rear ratio.

To determine the rear ratio, put the rear on jack stands. Put a piece of tape on the drive shaft and rotate the rear tires (by hand not under engine power) one complete revolution. Count the number of times the drive shaft turns over. Likely it will be in the lower 3:00's to 1. My car has 3:70's in it and the tach reads approximately 2700 rpm at 65 mph. Someone on this forum should be able to tell you what number tooth tranny gear to get for the rear end ratio that you come up with.

Putting the gear in is simple. Check the tail housing on the tranny. You should see the cable coming in from the speedometer head. Loosen the bolt that holds the head of the cable in and allow the keeper to rotate out of the way. Slide the cable end out and the gear that is presently in it should come out with the head. Slide that gear out of the cable head, replace it with the correct one, lube it up a little and reinstall it. Should be set to go from there.

The tach rpm sounds like it is reading correctly so I don't think there is any need to fool with it.

Good luck.

Steve

 

Thanks for the reply. Can I assume that the replacement part that I need is readily available from the ussual Corvette vendors or can I purchase at a local parts store?

 

There are 2 gears for the speedometer.

A Drive gear which is metal and is pressed into the transmission shaft. There are 2 of these: 1 for tall ratios and 1 for short ratios. I don't remember which rear end ratio gets which drive gear.

A Driven gear which is nylon and inserts into the side of the transmission.

The Driven gear is easy to replace. The metal Drive gear requires removal of the transmission.

Mixing the wrong Driven and Drive gears will result in a chewed up Driven gear. Guess how I know that...

 

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Jeff,

Never heard of this!!! Not saying you are wrong but what would the difference be? I had to replace a plastic gear in my 1970 Nova SS. Original gears were 3:55 and the guy who owned it before I got installed a set of 4:11's. I simply found the correct plastic gear and installed it to correct the speedo. Would it be a difference between manufacturers? Muncie vs Borg Warner? I guess I need some education on this.

Steve

 

Well, cable driven speedos are notoriously non-linear. In other words at low speeds they can be pretty close but as speeds increase the gap between real speed and readings get larger. This is especially true with worn speedometer heads. I took my vette to a speedo calibration shop (when the plastic gears did not correct the error) and for $100 they installed an adapter box down at the trans. All is well now.

 

Here's the chart from the parts manual. You don't need to know the axle ratio. You need to know the color and number of teeth on the driven speedo cable gear in your Muncie (Muncies have at least 2 different drive gears - noted on the chart). When you identify your current driven gear, select a new one (for the drive gear you have)with the number of teeth that will increase (fewer teeth) or decrease (more teeth) the speedo cable revolutions by the percentage error you see.

That avoids all the tire height problems you will cope with unless you have crusty old factory tires on the car.

ratio__tire size___drive gr_____driven gr_teeth_color
2.73__8.15-G70__3895494____3987918_18____brown
3.08__7.75 -15___3924098____3987918_18____brown
3.31__8.45-G70__3924098____3987919_19____natural
3.36__8.15-G70__3924098____3987920_20____blue
3.55__8.15-G70__3924098____3860344_21____red
3.70__8.15-G70__3924097____3860345_22____green
3.70__8.45-_____ 3924098____3860344_21____red
3.73__8.15-G70__3924097____3860345_22____green
3.73__F70______ 3924097____3860348_25____orange
4.10__8.15-G70__3924097____3860347_24____yellow
4.11__F70______ 3924097____3860348_25____orange
4.56__8.15-G70__3924097____3860345_22____green* uses 3890575 adapter

 

Magic,

Here is a couple of the parts that you listed. They indicate that the same gear, driven and driver, use the same number of teeth, same tire size, etc. for 3:73 and 4:11 rear ratio. How does that work???

3.73__F70______ 3924097____3860348_25____orange
4.11__F70______ 3924097____3860348_25____orange

Steve

 

I'm getting confused!!!

 

From my "Tech Bench" column in the April '06 issue of CE:


TECH BENCH

John Hinckley


Reader’s Question: I’m confused about speedo gear combinations for different axle ratios. When I bought my ’67, the speedo reading was way off (reads too fast), and the differential gears must have been changed; the stamping on the bottom of the (apparently-original by its date code) diff is “AL”, which is supposed to be 3.08:1, but marking the prop shaft and turning the wheels says it now has 3.55 gears in it. I can’t make sense of the “large-vs.-small” drive and driven speedo gears I’ve read about, and I need to know what I need to do to make it read properly; I removed the little plastic gear to check it, and it’s brown and has 18 teeth.

Response: It’s not unusual to find original diff housings with gear changes, and that’s apparently what you have. Here’s the story on speedo gears for ’63-’67 4-speeds and Powerglides.

Two different diameter 8-tooth steel speedo DRIVE gears were used, pressed on the output shaft, and each one used a different diameter “family” of nylon DRIVEN gears to accommodate a range of rear axle ratios.

The “large” DRIVE gear was 1.85” in diameter, and used the “small” (0.8” diameter) family of DRIVEN gears, as follows:

3.08 = 18-tooth, #3987918 (brown)
3.36 = 20-tooth, #3987920 (blue)
3.55 = 21-tooth, #3987921 (red)
3.70 = 22-tooth, #3987922 (silver)

The “small” DRIVE gear was 1.77” in diameter, and used the “large” (0.87” diameter) family of DRIVEN gears, as follows:

3.70 = 22-tooth, #3860345 (green)
4.11 = 24-tooth, #3860347 (yellow)
4.56 = 20-tooth, #3860329 (steel – used with special transmission with 6-tooth 1.77” diameter DRIVE gear)

For your 3.55:1 application, you need the #3987921 red 21-tooth DRIVEN gear in order for the speedo to read correctly. Fortunately, both your original and new axle ratios are within the same “large” DRIVE gear/”small” DRIVEN gear family, so a simple driven gear change will solve the problem. If one axle ratio had been in one family and the other axle ratio in the other family, you’d either need to change the pressed-on DRIVE gear to be able to use the correct DRIVEN gear, or you’d need to have a speedo shop make you up a ratio-adapter box.

The above assumes stock tire sizes (6.70-15 or 7.75-15) or replacements with the same diameter and tire revolutions per mile. For other tire sizes, you can also calculate the required tooth count for the driven gear; you’ll need to know tire revolutions per mile, which you can get from the tire manufacturer or develop as follows:

Tire Revs per Mile = 20168 divided by tire diameter in inches.

Once you have revs per mile, use this formula to derive driven gear tooth count:

DRIVE gear tooth count (8) x axle ratio x revs per mile, divided by 1001 = teeth.

(GM speedometers turn 1001 rpm at 60mph, or 1001 revolutions per mile)

Generally speaking, a 1-tooth change in the DRIVEN gear tooth count will result in a 5% change in the speedo reading (more teeth makes it read slower, less teeth makes it read faster).

’68-’70 4-speeds follow the same general gearing rules (although the DRIVE gears are nylon, retained on the output shaft by a spring clip), and ’71-up 4-speeds used ONLY the “large” DRIVE gear and “small” DRIVEN gear family; they used ratio adapter boxes for the limited number of 3.70 and higher (numerical) axle ratios offered.

 

OK, the 4.11 F70 is a typo. Here's the expanded list - you make sense out of it
year__ ratio_tire size____drive gear__driven gear_teeth_color
65-___2.73__8.15______unspecified__1359273_ 39____brown
67-___2.73__8.15______3895494____3987919_19 ____natural
68-___2.73__8.15-G70__3895494____3987918_18____brown
61-66_3.08__7.75-15___3924098____3987918_18____brown
67-68_3.08__7.75-15___3924098____3987919_19____natural
61-67_3.31__7.75______3924098____3987920_20 ____blue
67-68_3.31__8.45______3924098____3987919_19 ____natural
61-66_3.36__8.15-G70__3924098____3987920_20____blue
68-___3.36__8.15-G70__3924098____3987920_20____blue
67-___3.55__7.75______3924098____3860345_22 ____green
67-___3.55__8.15-G70__3924098____3860346_23____black
67-___3.55__8.15-G70__3924098____3860344_21____red
56-60_3.70_unspecified_unspecified__3708699 _22____green
61-64_3.70__7.75______3924097____3860345_22 ____green
67-___3.70__8.45-_____ 3924097____3860346_23____black
68-___3.70__8.45-_____ 3924098____3860344_21____red
67-68_3.73__8.15-G70__3924097____3860345_22____green
68-___3.73__F70______ 3924097____3860348_25____orange
67-___4.10__7.75______3924097____3860348_25 ____orange
68-___4.10__8.15-G70__3924097____3860347_24____yellow
56-60_4.11_unspecified_unspecified___370814 3_24____yellow
61-64_4.11__670-15____ 3924097____3860347_24____yellow
61-67_4.56__8.15-G70__3924097____3860329_20____steel
67-___4.56__7.75______3924097____3860344_21 ____red
68-___4.56__8.45______3924097____3860345_22 ____green uses 3890575 adapter
68-___4.56__8.15-G70__3924097____3860346_23____black uses 3890575 adapter
68-___4.89__8.15-G70__3924097____3860347_24____yellow uses 3890575 adapter

 

The key to all this is which circle gear in in the trans now, and "untouched by human hands" don't mean squat with these cars. Here is a list of what you can get now new.

Speedometer Circle Gear
L6T steel: 1.92" OD (1963-70) 3845079
L7T steel: 1.76" OD (1963-70)
L8T steel: 1.76" OD (1963-70) 3708145
L7T steel: 1.84" OD (1963-70)
L8T steel: 1.84" OD (1963-70) 3708144
L8T natural nylon: 1.84" OD (1969-70) 6261794

NOTE: these SIX (6) gears have 30mm bore: used with 27 spline mainshaft.

L8T steel: 1.84" OD (1971-74) 3978758 NOTE: this ONE (1) gear has 35mm bore: used with 32 spline mainshaft.

Speedometer Circle Gear Retainer
used w/ nylon gear only (1969-70) 6261781

Speedometer Pencil Gear
L20T steel 3860329 3860329
NOTE: this ONE (1) gear used with 1.92" outside diameter circle gear.
L22T green nylon 3860345
L23T black nylon 3860346
L24T yellow nylon 3860347
L25T orange nylon 3860348
NOTE: these FOUR (4) gears used with 1.76" outside diameter circle gears.

L17T purple nylon 3987917
L18T brown nylon 3987918
L19T natural nylon 3987919
L20T blue nylon 3987920
L21T red nylon 3987921
L22T silver nylon 3987922
NOTE: these SIX (6) gears used with 1.84" outside diameter

Now all you need to know is the gear out back and the diameter of the tire, then you compute the driven gear.

I have or can get all these parts, most I have.

 

That's a whole lot of information to digest. I guess I better try to determine my rear end ratio first. As always, forum members came to my rescue. Thanks guys!!!

 

How do you know if you have the Large drive gear or small drive gear. I have a 3.70 which you have listed under both drive gears. I have a 65 with a L79 CR 4 speed. Tires are repro gold lines. Thanks for your help.

 

That is the question. You can't rely on the trans being original. So the only real way to determine the OD of the circle gear without taking it out of the car, is to pull the driven gear and measure it. Look it it as well and make sure it's not chewed up, a clear indication that some idiot has put the large diameter driven gear on the large circle gear.

 

After you determine the gear ratio, post that ratio and tires size/height and someone can be more specific on the speedometer gears.

Verle

 

The "large" and "small" DRIVE gears on the output shaft are different thicknesses, and you can check it with a gage made from cardboard or sheet stock, through the speedo adapter hole. I'm out of town for the weekend and don't have access to my notes right now - try me again on Monday and I'll post the two thicknesses.

 

This may be a stupid question, but are all speedo cables on the same side? Mine is on the passenger side. My engine is a 69 350. I put the car on ramps this afternoon. The speedo cable was actually loose. I only needed my fingers to remove it. I could not remove the plastic gear drive. Is there a trick to it? Actually I was pressed for time and had to quit. I'll be back at it tomorrow. One thing that I did notice was that I was missing two nuts from the drivers side exhaust manifold.

 

Speedo gear is on the right side of my 65.

Did you remove the bolt that retains the metal housing that contains the plastic gear?

Important - be sure to disconnect the speedo cable before removing the metal housing for the plastic gear, otherwise as you remove the metal housing the plastic gear may get pushed into, and fall in, the transmission case.

 

The "small" DRIVE gear (1.750" diameter) is .531" thick, and the "large" DRIVE gear (1.85" diameter) is .610" thick.

63's and 64's used the 429 tailhousing with the speedo on the driver's side, and '65-up used the 584 tailhousing with the speedo on the passenger side.