Running a 3.08 rear end behind a TH 400 which I believe is a 1:1 as a 3 speed.
Is there a way to calculate or know the proper speedo gear color/tooth count for that overall set up?

 

https://www.5speeds.com/calculators/SpeedoCalc.html

 

Not without also knowing the tire diameter.

 

The tire is a 15" but where I'm lost is the drive and driven gear.
I have no idea what the drive gear tooth count is.
The chart that woodie put up shows a range of tooth counts and leave me even more lost on this.

 

No, the WHEEL is 15". Unless you are driving around on the metal rims, that is irrelevant. What matters is the outside diameter of the tire.

 

There is a speedo gear calculator on TCI transmission FAQ. As Joe mentioned, you need to know the axle ratio, tire diameter, and preferably either the drive or driven gear tooth count. Enter that info, it will calculate the other gear needed.

Sometimes you will come up with drive or driven gear that has no match for your tire/gear ratio. When that happens you need to play around with changing both gears to come up with an acceptable combination. Furthermore, you need to make sure the driven gear will fit the existing housing, or make sure you order the correct part.

 

Unfortunately sometimes even that doesn't work, which is why GM occasionally had to use ratio adapters from the factory.

 

Still can't wrap my head around this.
The tire od is 26"
The calculator asks for the drive gear tooth count of which there could be 9 different gears.
I take it the driven gear housing must come out to see the color to know that.
I think the problem is the original rear end was 342 with a 14" tire.
Now its a 308 with a 15" (26 od) and the speedo reads about 10 mph higher.
Then we have 19 driven gear possibilities....
Lastly what is Aspect Ratio and how does it factor into this?

 

You need to determine which drive gear is currently in your trans. They are color coded, but it may be discolored from heat and age and red trans fluid. Pull the speedo fitting out of the trans and look in. If you can determine the color, you're that much further ahead. Aspect ratio of the tire is just one way to determine the actual outside diameter. Are you SURE it's exactly 26"? As an example, 235/60-15 is actually 26.10 inches. 60 is the aspect ratio. What is your actual tire size?

 

Yep, I have a few of those adapters from core transmissions. I’m pretty sure there are speedometer shops that can make custom ratio adapters.

 

This example needs a driven gear with 42.5 teeth. Obviously you can’t have a gear with half a tooth, either a 42 or 43 is needed

Changing the drive gear gets the needed driven gear closer to a whole number.

The closer to a whole number the more accurate the speedo.

 

Thanks Joe, that clears my head on this.
My blind shot in the dark would have been a common gear used in these cars, and yes no guarantee the trans is original.
Its going on the lift for a trans filter change, I will dig in when that happens.
Never would have thought there would be that many drive gears for the number of driven gear combinations honestly.
Thanks for the help.

 

Consider that the TH400 was used in everything from F-body cars with small diameter tires to two ton trucks with very tall tires, and with rear axle ratios from 2.14:1 to 5.00:1 (and above for the big trucks).

 

Any way to know how the trans was tagged to know if its a match as original?
THat asked, would there be an expected drive gear count as a 400 in front of a 342 rear end?

 

There's a straight-forward way to determine the tooth count you need...

1. count the number of teeth on the driven gear in the trans.
2. drive at exactly 60 mph as indicated on your speedometer.
3. have someone simultaneously use a smart phone GPS app to determine your actual speed.
4. divide 60 by the GPS speed and multiply by the number of teeth on the existing driven gear.
5. the result is the number of teeth you need on a new driven gear.

If that number of teeth is off the chart, you will need to follow Joe's advice to select a different drive gear or buy an in-line gear-box adapter.

 

The ID tag on the passenger side of the trans will tell you where it originally came from, and the VIN derivative stamp on the driver's side will tell you if it's original to the car or not. In any case, the original rear could have been 2.56, 2.78, 3.08, 3.23, 3.42, 3.91, 4.33, or 4.66 ratio. The Product Information Manual says that for 3.08 gears, the drive gear in the TH400 was 18 teeth.

 

Great info
So would that mean that in order for this 308 to read proper speed I'd have to change the drive gear in the transmission?
Or would it reason that once I know the drive gear tooth cound I could find the correct or close driven gear with the calculator?

 

Depends on how far off the current drive gear is. If it's close, you can probably make due with a different driven gear, possibly with a ratio adapter. There's more than one way to get to the right combo.

 

Probably not. If you have the standard 18-tooth drive gear, you can get driven gears with from 34 to 45 teeth. That's a big enough range that just changing driven gear would normally suffice.

You would change the drive gear or get a ratio adapter only if your calculation shows that you need a driven gear with a number of teeth outside the 34 to 45 availability.

If you're still unsure, post the color (or tooth count) of the existing drive and driven gears along with your GPS and speedo readings. We'll help you select the correct gear.

 

Here’s a nice estimator, from an old Gratiot Auto Supply Mag, nice bonus tricked out F body there, lol

 

Finally getting back to this with the car now up.
The drive gear is coded with yellow paint.
Is there is chart that shows the teeth count coded yellow?
The driven gear is orange and marked 35T.
Speedo is running about 10 mph fast.
Kinda glad I went there because I found the speedo cable nut cracked in 3 places.
She is pretty grimy but hasn't given me a reason to pull and overhaul it.
Last engine was leaking pretty badly.

 

Orange driven gear checks out at 35 teeth as you indicate.

You will need to put in a Yellow driven gear (41 teeth).

The drive gear does not need to change and you do not need a ratio adapter box.

Your speedo will read 0.4% (one-quarter of an mph) slow. But this accuracy depends on how carefully you measured your speed at 60 mph indicated.

Your statement "speedo is running about 10 mph fast" could indicate that you didn't use GPS or stopwatch. You also didn't state at what indicated speed you checked actual speed.

 

My speed differene of 10 mph came from WAYS vs speedo read.
When the speedo reads 75, I am close to 65 mph.
Thanks for the help, 4% is close enough for me.

 

You will have to change the speedometer gear housing to use a 41 tooth gear.
one housing is for 34 to 39 teeth and the other is 40 to 45 teeth.

 

Thanks for the heads up Nick.
Finally coming together now.

 

With that additional information, you should use a Black (40 tooth) driven gear.

Using the Black gear, your speedometer will read about 59.4 mph at actual 60 mph.

 

Even better to be closer to correct thank you.
I was able to cancel and reorder the housing and 40T in time.
ty

 

Hey Joe,

May I revive a 9 month old post on which you were assisting a member in selecting the proper speedometer gear, please? My situation is that I purchased a '69 4477 with an M20 and factory A/C. That tells me that it should have come with a 3:23 axle ratio (it is open); however, the transmission, which was divorced from the car when I purchased it, did not have the housing or a gear in it. I am running a set of Firestone G70x14 Wide Ovals (not radials, but bias belted) tires on the car mounted on steel 14" stock rims. What are your thoughts on the correct teeth count/color for the speedometer gear in this car?

Thanks!

Buddy

 

3.23 was standard with M20 but 3.08 was optionally available, so be sure you know which you have. G70x14 tires are 26.82" outside diameter. This was not an available tire size in 1969, but the 7.75x14 tires were 26.70" so pretty close. For that size the factory used a 25 tooth driven gear and a 1.2720 ratio adapter external to the trans.

 

Yes, I knew that the F70 was the correct size for the '69 Wide Oval, so is the external adapter necessary to compensate for the difference in tire size? I've found a 25 tooth gear, but finding the external adapter for that particular ratio is a little more challenging. If I don't install the adapter, I'm assuming the speedometer reading will be off by what...15-20%?

Thanks Joe!

 

The adapter was factory installed with the 26.70" tall tire. The speedo will be off by 27.2% (it's a 1.272:1 ratio adapter). There are a ton of aftermarket adapters on the market if you don't care about originality.Google speedometer adapter

 

Thanks so much for your assistance Joe!

Buddy

 

Buddy, if your car is driveable, post 15 shows another way of correcting the speedometer error. Use whichever method is easier for your situation.

Gary

 

Thanks Gary, but it's not currently driveable. Probably 60 days out from that. It didn't have a gear, housing or seals so it pukes trans fluid when running.

Buddy