Need help choosing a speedo gear for my 60

[Greg L.]

I need to switch gears here and work on my 60 for a while....

The speedo in my 60 reads at tad low as in it shows about 50 mph when it's really doing about 62(taken from a GPS). The odometer also shows about 9 miles for every 10.2 miles driven(compared to my 03 GMC)but that's ballpark.

To me, I need to spin the cable a little faster but I'm not sure how much faster. I don't know what speedo driven gear is in it now but I have a 3:36 final drive and stock size tires.

I don't see that ratio listed for a 60 and I don't have any of my books with me but I'm thinking that if I can find the proper driven gear for that ratio for a 62 T-10 then I'll be okay so if someone could help me figure this out that would be great.

[Mike E.]

Re: Need help choosing a speedo gear for my 60

Don't have my books here, but first you need to pull the plastic gear in the tranny and see how many teeth are on it. Do your best guess on whether you need one more tooth or two, and order the gear/s and install one and see how close it comes. Looking at the book and figuring out what is the correct gear for a 62 3.36 ought to work, too. Final drive on both, in 4th gear, is one to one. Metal drive gears on the tranny output shaft to drive that plastic gear should be the same, unless your tranny originally came with a 4.11 or higher (as I recall).

[Greg L.]

Re: Need help choosing a speedo gear for my 60

Thanks Mike,

I was hoping to be able to figure out the proper gear based on my final drive ratio so that I could order it now and have it arrive home when I get home from vacation. I never though of removing the old gear until I had already left home.

[John H.]

Re: Need help choosing a speedo gear for my 60

Thanks Mike,

I was hoping to be able to figure out the proper gear based on my final drive ratio so that I could order it now and have it arrive home when I get home from vacation. I never though of removing the old gear until I had already left home.

Greg -

A 3.36 axle with stock (27.1" diameter) tires usually takes a 20-tooth speedo driven gear.

[Greg L.]

Re: Need help choosing a speedo gear for my 60

Thanks John,
I was guessing on the 20 tooth(blue) one but wanted a second opinion before I ordered. I'll go with that one.

[William C.]

Re: Need help choosing a speedo gear for my 60

Or do it the correct way, pull the dash and have the speedo cleaned and recalibrated so it will match the odo.

[Duke W.]

Re: Need help choosing a speedo gear for my 60

Hmmm.
Your speedometer is reading 24% low. (62 div by 50 = 1.24)
Your odometer is reading 13% low. (10.2 div by 9 = 1.13).

If both of these calculations, and variations are correct, you have problems in your speedometer head. Most likely with the speedo springs and air gaps. (There are recent threads on this subject).

You're going to go with a 20 tooth gear. If the existing one is 16 teeth (25% increase), you'll fix the speedo, but the odo will be 12.5% high. If the existing one is 18 teeth (10% increase) , you'll fix the odometer, but the speedo will be 14% under. You can't fix both with a gear change.

You also need to know what the old driven gear really is, and that the drive gear is compatible with the new driven gear. Then maybe pick a driven gear count between the two percentages needed to get a "happy medium".

The calculation goes like this:

Speedo: 50/62 -1 = -0.19 or 19 percent low

Odo: 9/10.2 - 1 = -.118, call it 12 percent low

There is a significant error difference between the speedo and odo, which may indicate something amiss other than just the speedo driven gear.

The OP should remove the driven gear, note the color and tooth count, then note the color/material and tooth count of the drive gear inside the gearbox.

If it is the correct drive gear, driven gear, the installed tires are about 760 revs/mile, and it actually has a 3.36:1 axle then a 20 tooth driven gear should be correct, but there may be problems in the speedo head.

Duke

[Greg L.]

Re: Need help choosing a speedo gear for my 60

You guys are right in that there are probably two issues with the speedo being out more than the odometer.

The way I figure it, get the odometer reading proper based on the final drive ratio first because that is a direct connection and then see what happens with the speedo. If it's still out alot then it looks like a speedo calibration.

I should have taken the gear out to have a look at it before going on vacation but I didn't so I hope the 20 tooth I ordered for when I get back will be the right one.

[Greg L.]

Re: Need help choosing a speedo gear for my 60

Update: The gear that was in the transmission was a black 23 tooth one. I replaced it with the new blue 20 tooth one. I noticed that the blue one was a little smaller which I figured was okay since there was less teeth but now the speedo doesn't work at all.

I'm not sure what the deal is now. Could it be the wrong gear or do I have to change the drive gear on the output shaft as well(put a bigger one on to make up for the smaller gear?

The transmission is a 62(I think) T-10 and that's what the blue gear was ordered for. It looks and fits like the old black gear except that the gear end diameter is smaller so it's like the teeth no longer mesh...

I've never had to experiment with speedo drive gears before so I'm not really sure what to do next.

Any ideas?

[Terry D.]

Re: Need help choosing a speedo gear for my 60

Greg
The 23 tooth would have been for a 4:11 rear end. What was the part number for the new gear? I believe there is a difference in the tranny gear, but off the top of my head I don't remember were the difference occurs. Not at home so I can't look it up. Some one should chime in and clear this up. I would rather have the speed correct rather then the od.
Terry

[Joe L.]

Re: Need help choosing a speedo gear for my 60

Update: The gear that was in the transmission was a black 23 tooth one. I replaced it with the new blue 20 tooth one. I noticed that the blue one was a little smaller which I figured was okay since there was less teeth but now the speedo doesn't work at all.

I'm not sure what the deal is now. Could it be the wrong gear or do I have to change the drive gear on the output shaft as well(put a bigger one on to make up for the smaller gear?

The transmission is a 62(I think) T-10 and that's what the blue gear was ordered for. It looks and fits like the old black gear except that the gear end diameter is smaller so it's like the teeth no longer mesh...

I've never had to experiment with speedo drive gears before so I'm not really sure what to do next.

Any ideas?

Greg-----


The black 23 tooth driven gear and the blue 20 tooth driven gear use different speedometer DRIVE gears. So, assuming that the 23 tooth driven gear functioned ok (except, of course, for calibration), you cannot install a 20 tooth blue gear and expect it to work with the installed DRIVE gear.

As you noted, the 20 tooth gear is smaller OD than the 23 tooth. While it MIGHT mesh for awhile. it will eventually "have the teeth shaved off" by the DRIVE gear.

The lowest tooth count DRIVEN gear that you can use with your installed DRIVE gear is a 22 tooth GREEN gear.

If you need fewer teeth than that for proper calibration you will have to either change the DRIVE gear or install a speedometer adapter to achieve correction.

[Tom P.]

Re: Need help choosing a speedo gear for my 60

Greg-----


The black 23 tooth driven gear and the blue 20 tooth driven gear use different speedometer DRIVE gears. So, assuming that the 23 tooth driven gear functioned ok (except, of course, for calibration), you cannot install a 20 tooth blue gear and expect it to work with the installed DRIVE gear.

As you noted, the 20 tooth gear is smaller OD than the 23 tooth. While it MIGHT mesh for awhile. it will eventually "have the teeth shaved off" by the DRIVE gear.

The lowest tooth count DRIVEN gear that you can use with your installed DRIVE gear is a 22 tooth GREEN gear.

If you need fewer teeth than that for proper calibration you will have to either change the DRIVE gear or install a speedometer adapter to achieve correction.

Joe,
I'm glad you said that. I've been reading this and thinking that these guys don't understand what's going on inside a tranny with the speedo gears.


Greg.
For the manual trannys (3sp, T-10, Muncie) there are TWO sizes of drive gears (the one inside that's pressed onto the mainshaft) and two sizes of driven gears.
MOST often, the drive gears that are installed have 8 teeth----------------but there are other sizes (7 or 9) that fit--------------I have RARELY seen them.
The driven gears come in 17-25 teeth versions. The 17-22 teeth driven gears are smaller and must be used with the large drive gear.
The large driven gears come in 22-25 teeth versions and must be used with the small drive gear.
If you install a large driven gear (22-25 teeth) with a large drive gear, it will be chewed up real quick.
If you install a small driven gear (17-22) with a small drive gear, it won't work--------------------------as you discovered.
Now I mentioned a large and a small 22 teeth gear. Yes, that's right. Green large, plain small.
So, as Joe pointed out, your options are to either pull off the tail housing and install a large drive gear (that would be my choice). The large drive gear has an OD of 1.84in, the OD of the small one is 1.76in. Just barely enough difference to keep the drive/driven gears from being compatable.
Your second option (and kind of pricy) is to obtain a speedo adapter (they are available in both a reduction or increase in speed of the output side of the adapter) which just screws onto the existing speedo fitting on the tranny. The second option is certainly the easiest, BUUUUUUUUUUUUUT, it may be a hassle as well as interfere with the shifter linkage.

[Greg L.]

Re: Need help choosing a speedo gear for my 60

Thanks guys. I figured it was something like too small a drive gear but you guys confirmed it.

I've seen those speed adapters and I don't think there will be enough room with the linkage so that's out. Replacing the drive gear is probably the way to go but I won't be doing that this year. OR, I guess I could put in a set of 4:11s but I'll probably need a different carrier because I don't think the 3:36 one is compatible with the 4:11 one...

I talked to my Dad the other night about this and he remembers that when he worked at the dealer that sometimes they would install an aftermarket OFFSET driven gear housing. The bore that the shaft of the driven gear rides in was actually slightly tilted either towards or away from the drive gear the required amount for the different gear sizes so that the gears would mesh properly. Does anyone remember these housings and are they still available?

[Tom P.]

Re: Need help choosing a speedo gear for my 60

Never seen nor heard of such an animal (but that doesn't mean such a thing doesn't exist). I've been rebuilding 3 and 4spds for nearly 50yrs.