[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.

[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?