[GordonGeesey]

Need some assistance figuring out which speedometer gear I need:

My car ('69 GTO) was originally an automatic with the special order 3.90 gear set. The original rear/gear set is still present, but the transmission was switched to an M-20 4-speed manual long ago by a previous owner. I am running original size G70-14 tires on 14x6 rims.

The speedometer reads 50mph while my GPS app says that my actual road speed is 40mph. Close enough to call it a 20% difference, and obviously not close to being accurate.

I have NOT gone into the trans yet to see what color gear I have or to count the teeth.

I found the attached chart on line. It looks pretty simple, but I don't understand what "Drive Gear Teeth" means?? Also, the chart says my correct gear should be green, but the 18-tooth (GM) gear sold by AMES is color-coded as brown. Does the color really matter, or just the tooth count?

Can anyone get me pointed in the right direction to fix this issue?

TIA !!

[Greg Reid]

Thanks for posting that chart. I've been living with the same problem for years. About time to fix it so looking forward to the responses.

[north]

double entry

[north]

The color of the driven gears was just for easy identification. The driven gears are the pinion type plastic gear that slides in where the speedo cable goes. The drive gear is a ring gear on the output shaft. The drive gear is a bigger job to change, any M20 should have 7 teeth. The original set up for a car with an M-21 (M20 wasn't offered with 3.90 gears) and 3.90 gears is 6 tooth drive and 18 tooth driven which means you are spinning the cable exactly 3 times slower than the drive shaft. Most A, F or G body 69's I've tested with the correct original speedo gearing run 2-4% high from the factory. So reading 20% high is really say 17% high especially with 70 series tires instead of 78. This makes perfect sense since you are running an M20 which likely has 7 drive teeth, you are probably already running a brown driven gear (18 tooth) which would give you exactly the result you see with your GPS.

The problem you have is that you need a 21-22 inch tooth driven gear which GM didn't make for muncies. A 20 tooth driven will fix half your problem, the best solution is to order a 6 tooth drive gear (typically used on M-21) and keep the 18 tooth driven in place. The only bummer is to change the drive gear you need to open up the back of the tranny (tail housing) to get at it.

[Greg Reid]

Is the FO trans code referring to a TH-400 or what?

[north]

FO is M20 and FT is M21

[north]

All the auto trans codes started with P, PX for base GTO engine and PQ for either Ram Air engine. PT was used on the 2bbl engine option. DB was the code for the standard 3 spd manual trans.

[north]

3.90 or 4.33 cars with a TH400 trans didn't use a different lower tooth drive gear, instead they used an external adaptor (which was mandatory option 591 if you ordered either of those gears with a TH400).

[Greg Reid]

Ok, got it. Thanks.

[GordonGeesey]

Originally Posted by north

The color of the driven gears was just for easy identification. The driven gears are the pinion type plastic gear that slides in where the speedo cable goes. The drive gear is a ring gear on the output shaft. The drive gear is a bigger job to change, any M20 should have 7 teeth. The original set up for a car with an M-21 (M20 wasn't offered with 3.90 gears) and 3.90 gears is 6 tooth drive and 18 tooth driven which means you are spinning the cable exactly 3 times slower than the drive shaft. Most A, F or G body 69's I've tested with the correct original speedo gearing run 2-4% high from the factory. So reading 20% high is really say 17% high especially with 70 series tires instead of 78. This makes perfect sense since you are running an M20 which likely has 7 drive teeth, you are probably already running a brown driven gear (18 tooth) which would give you exactly the result you see with your GPS.

The problem you have is that you need a 21-22 inch tooth driven gear which GM didn't make for muncies. A 20 tooth driven will fix half your problem, the best solution is to order a 6 tooth drive gear (typically used on M-21) and keep the 18 tooth driven in place. The only bummer is to change the drive gear you need to open up the back of the tranny (tail housing) to get at it.

Great info - thanks! AMES lists a 21 and a 22 tooth driven gear for a Muncie in their catalog. Will one of those solve my issue w/out having to open up the trans and change the drive gear?

[north]

Good question, you have to wonder why GM felt it necessary to have different drive gears. I’d call Ames and confirm they are compatible (tooth angle) with your drive gear.

[Greg Reid]

Bringing this back up as I'm trying to select a driven gear for my car. Is there a way to tell the of drive teeth in the transmission without getting the car up on the jack and looking? I mean, how many variations were there in the TH-400?
I'm trying to use the Ames calculator but I'm missing that part of the formula. Of course, if I have to I'll get under there and check.

[Greg Reid]

Seems that 15 is typical?

[north]

Typically there were two or maybe three possible drive gears. If you have a shop manual or MPC illustration section you can look it up by the transmission ID code.

[north]

Teeth count on drive gear was usually 6 or 7.

[Greg Reid]

Thanks north. Sounds like I need to get under there. Well, I was going to have to get under there to change it anyway. I was hoping that maybe there was just one standard size for the autos then I could have just ordered the driven gear I need.

[Greg Reid]

We were typing at the same time. Using the formula I just took a wild guess and put in six and seven but the number of teeth on the driven gear was something like 17... The smallest number of teeth on the driven gear Ames has for automatic transmission is 34..
All of the turbo 400 drive gears that I'm finding for sale online have 15 teeth. That comes out to 39 teeth on the driven gear and that's in the middle of the pack for driven gears offered by Ames.

[north]

They rarely are ever changed so if you have the year and two letter code of the th400 you can look up what it came with. Chances are it’s still the original gear.

[Greg Reid]

I can't recall the code. I looked at it when I first got the car but that was 18 years ago. I may have written it down so I'll check that. I guess. if I have to get down there to get the code it may be just as easy to look at the gear. Not sure how easy it is to see that since I've never messed with speedo gears.

[north]

Worth trying to read the code if the alternative is seeing the drive gear (tail shaft has to come out). If you are trying to see the driven gear just unscrew the speedo cable and then pull out the gear