Speedometer Drive Gear. 20 Tooth Blue

[Irby G.]

Could someone please explain the difference (besides the obvious numbers of teeth) between the 20 -25 tooth speedometer gears? Does this have anything to do with the type ratio you're using? I believe I'm running 3.70 with close ratio on my '63 340.

[Joe L.]

Re: Speedometer Drive Gear. 20 Tooth Blue

Could someone please explain the difference (besides the obvious numbers of teeth) between the 20 -25 tooth speedometer gears? Does this have anything to do with the type ratio you're using? I believe I'm running 3.70 with close ratio on my '63 340.

Irby-----


The correct speedometer driven gear for an application depends on several things. These include rear ratio, type of transmission, installed DRIVE gear, size of tires and more.

The 18-22 tooth (silver) speedometer DRIVEN gears are the "small" OD gears. They are GENERALLY used with rear ratios of 3.55:1 or numerically less. The 22 (green)-25 tooth DRIVEN gears are the "large" OD gears. They are GENERALLY used with 3.70:1 and numerically higher rear ratios.

If you tire OD is the same as stock, I would expect you'd require a 22 tooth GREEN gear of GM #3860345. This assumes that the transmission is original and/or has the correct speedometer DRIVE gear installed.

[Chuck G.]

Re: Speedometer Drive Gear. 20 Tooth Blue

Could someone please explain the difference (besides the obvious numbers of teeth) between the 20 -25 tooth speedometer gears? Does this have anything to do with the type ratio you're using? I believe I'm running 3.70 with close ratio on my '63 340.

All other things being equal, a gear with fewer teeth will spin faster, and turn the speedo faster= higher MPH readings.

A gear with more teeth will spin slower=lower MPH readings.

I don't have the charts handy, but many of the Corvette catalogs and also the AIM's have them showing the correct number of teeth for a particular rear end ratio.

This assumes that you're using tires of the same diameter as stock.

Close vs. Wide ratio in the trans doesn't matter, as 4th gear in the trans is 1:1.

Chuck

[Wayne M.]

Re: Speedometer Drive Gear. 20 Tooth Blue

Could someone please explain the difference (besides the obvious numbers of teeth) between the 20 -25 tooth speedometer gears? .

Irby -- gives me the excuse to post this still-life .

[Stuart F.]

Re: Speedometer Drive Gear. 20 Tooth Blue

Now that is a beautiful piece of art.

Stu Fox

[Tom P.]

Re: Speedometer Drive Gear. 20 Tooth Blue

There are TWO sizes of BOTH the drive and driven gears.
The drive gears (pressed onto the tail shaft) come in OD of 1.76in and 1.84in.
The plastic driven gears, as previously mentioned, also come in two sizes, and must be used with the correct corrosponding size drive gear.

If the appropriate dirve and driven gears are not matched, they will not mesh, or, they will not mesh properly and the end result will be a chewed up driven plastic gear.

Use the large (1.84in) drive gear with the 17-22teeth small plastic gears.
Use the small (1.76in) drive gear with the 22-25teeth big plastic gears.

In the 69 Muncies (maybe some 68 and 70 also), they used a plastic drive gear that was retained on the tail shaft with a metal spring clip. It is my strong recommendation when you rebuild your Muncie, if it has the plastic drive gear, to replace it with a pressed on steel gear. It is very common for the teeth on the plastic drive gear to wear excessively, and to replace it requires partial disassembly of the tranny, whereas replacing the driven gear is a snap to do.

The small plastic driven gears are (use with 1.84in drive gear):
3987917-purple-17teeth
3987918-brown-18teeth
3987919-neutral-19teeth
3987920-blue-20teeth
3987921-red-21teeth
3987922-gray-22teeth

The large plastic driven gears are (use with 1.76in drive gear):
3860345-green-22teeth
3860346-black-23teeth
3860347-yellow-24teeth
3860348-orange-25teeth

[Irby G.]

Re: Speedometer Drive Gear. 20 Tooth Blue

Thanks everyone...
Tom, After reading what everyone posted, I was just about to ask a bunch of questions and you covered them all in one post.
Wayne, Very nice pic.

[Wayne M.]

Re: Speedometer Drive Gear. 20 Tooth Blue

Thanks everyone...
Tom, After reading what everyone posted, I was just about to ask a bunch of questions and you covered them all in one post.
Wayne, Very nice pic.

Irby -- One more pic to show the difference on the two 3.70 ratio 22-tooth gears that Joe and Tom list. Hard to notice the diameter difference for the same tooth count, but the green is sligtly larger. Depending on your drive gear, you can select the proper driven gear that will give the same rotation speed for the cable.

Was not always this way; the gray (silver ?) gear was introduced post-1970 (Joe will probably know when), probably to satisfy service complaints.

So you nth-degree NCRS purists should NOT use the gray gear. Who knows ? in another 20 years, a multi-star Bowtie might call for unscrewing the speedo cable from the trans tail and checking color of plastic driven gear .

[Tom P.]

Re: Speedometer Drive Gear. 20 Tooth Blue

Irby -- One more pic to show the difference on the two 3.70 ratio 22-tooth gears that Joe and Tom list. Hard to notice the diameter difference for the same tooth count, but the green is sligtly larger. Depending on your drive gear, you can select the proper driven gear that will give the same rotation speed for the cable.

Was not always this way; the gray (silver ?) gear was introduced post-1970 (Joe will probably know when), probably to satisfy service complaints.

So you nth-degree NCRS purists should NOT use the gray gear. Who knows ? in another 20 years, a multi-star Bowtie might call for unscrewing the speedo cable from the trans tail and checking color of plastic driven gear .

And inline with this same level of judging, when are they going to start pulling the camshaft out of engines to see if it has the correct casting number on it?
Or when will they start pulling rears out to check if the stamped numbers (part number, date code, gear ratio) on the pinion head match the stamped code on the center case?
And is there an air quality analyzer available that can be connected to the valve stems to analyze the air inside the tires to determine if it actually came from the St. Louis area.

[Wayne M.]

Re: Speedometer Drive Gear. 20 Tooth Blue

There are TWO sizes of BOTH the drive and driven gears.
The drive gears (pressed onto the tail shaft) come in OD of 1.76in and 1.84in.
The plastic driven gears, as previously mentioned, also come in two sizes, and must be used with the correct corrosponding size drive gear...

Tom -- you're neglecting my favorite production gear set; the 4.56's

Drive is 6-tooth (not 8 like the others) and mates with a steel 20-tooth driven (and maybe a few others), from 1961 thru 1966, with stock tires. Drive is Gr 4.343 #3845079, 1.922" OD by 0.453" thick, by my caliper measure. Driven is Gr 4.337 # 3860329.

[Mike M.]

Re: Speedometer Drive Gear. 20 Tooth Blue

Wayne,
that is a really cool shot.
Could I please have at least a 20" X 25" or larger, on acid free paper with the UV coating?
My wall has the perfect location for that.
Thank you and HaND
PS: if we could download photos, I could save you hassle.

[Mike M.]

Re: Speedometer Drive Gear. 20 Tooth Blue

Well Tom, with all of the speed traps here, I would just like to know what my driving speed is.
Secondly, if some of the owners here wish to take their car the distance, ie: PV, this information is critical and quite timely.
As part of a PV in Kissimmee last Thursday, a road test is required, part of which is to confirm speedo/odometer accuracy.
A pre-designed and measured course is used.
One of the candidates had a speedo mis-calibration of -.4 at the halfway point and -.8 upon return.
Naturally, the owner asked whated to know the cause of the problem and how to fix it.
After of our brief verbal explanation of this (as above) and many other "Fixes" with the owner, he was unaware of the TDB, I suggested that he access the site for this type of valuable information.
Hopefully, he may be looking in as we speak.
HaND

[Mike M.]

Re: Speedometer Drive Gear. 20 Tooth Blue

Hi Tom, I get your point.
FWIW on another thought regarding the cam comment.
As judges, we are not concerned with the casting numbers, however, the owners, (Handlers, Mechanics, whatever) probably should be.
The first item on the PV is "Cold Start". Some PV's have ended before they began due the "Cold Start" not achieving the appropriate "High Idle".
The consensus of several Judges and Mechanics with later comments was that the cam was too wild to sustain the "High Idle".
HaND

[Joe L.]

Re: Speedometer Drive Gear. 20 Tooth Blue

Irby -- One more pic to show the difference on the two 3.70 ratio 22-tooth gears that Joe and Tom list. Hard to notice the diameter difference for the same tooth count, but the green is sligtly larger. Depending on your drive gear, you can select the proper driven gear that will give the same rotation speed for the cable.

Was not always this way; the gray (silver ?) gear was introduced post-1970 (Joe will probably know when), probably to satisfy service complaints.

So you nth-degree NCRS purists should NOT use the gray gear. Who knows ? in another 20 years, a multi-star Bowtie might call for unscrewing the speedo cable from the trans tail and checking color of plastic driven gear .

Wayne-----


The 22 tooth silver (or, gray) driven gear, GM #3987922, was introduced for the 1971 model year. Here's why: the 1971-74 Muncie (and, later Super T-10) had only one size available DRIVE gear. For the 71-74 Muncies the gear was GM #3978758 and for the T-10 it was GM #361002. Both of these gears are 8 tooth, 1.84" OD gears and will work only with the 0.80" OD DRIVEN gears (i.e. the 18-22 [silver] tooth).

So, for 1971 and later, the only way that higher numerical rear gear ratios could be accommodated was through the use of a speedometer adapter. In order to minimize the use of the rather costly adapters, the 22 tooth (silver) DRIVEN gear was added to the 0.80" OD series of gears. In most cases, this allowed rear ratios of up to 3.70:1 to be accommodated without the use of an adapter.

Notwithstanding the above, for some reason that I've never been able to figure out, some 1971 and later Corvettes with rear ratios numerically lower than 3.70:1 were originally built with an adapter.

[Wayne M.]

Re: Speedometer Drive Gear. 20 Tooth Blue

Wayne-----


The 22 tooth silver (or, gray) driven gear, GM #3987922, was introduced for the 1971 model year. Here's why: the 1971-74 Muncie (and, later Super T-10) had only one size available DRIVE gear. ....

Joe -- thanks for the detailed (as usual) explanation for the introduction of the silver gear. My newest catalogs, P&A30B in this case, are Rev. 1.1.70. So the additional plastic gear just "happened" to benefit C2 and early C3 owners that wanted to fine-tune speedo accuracy.

In this 1970 catalog of mine, under group 4.337, are 3 tables (each take up a page or more) of speedometer gear charts that cover the comlete range of final drive ratios, with different tire sizes, with all available transmissions, for the model years 1968, 1969 & 1970. These show drive and driven gears and adaptors, where necessary. I suppose you have this information as well.