BSiegPaint

My speedo has been off since I bought this car, and I did all the research and looked through as many threads as I could find (and the CSM) before understanding I needed a blue 20-tooth driven gear to correct things - except it didn't work. The car had the white 19-tooth in it when I changed the driven gear to blue - but the problem got worse. My speedo reads about 25% SLOW, but when doing calculations on what tooth gear would be my best choice (3.23 rear, 225 70R14 tires), the driven gear I would need to read correctly doesn't exist (15-tooth). The only thing I can figure is that the last person to touch this trans put in the wrong drive gear.

In all my searches, I recall one post where a member said it was possible to change the drive gear without removing the trans from the car. If this is true, I'm all ears if someone will point me in the right direction. I can't find that thread again for the life of me. Otherwise, I've ordered a new 17-tooth gear (fewest I can get) so that I will get a little closer to the proper speedometer reading. I'm so close to getting this car done the thought of dropping the trans makes me a little ill.... What are the procedures for just replacing the drive gear? Everything I see or read does it as part of a whole rebuild. I just need a new gear.

For reference, here are my speedo readings and actual GPS speeds: 20 = 27, 25 = 33, 30 = 39, 35 = 45, 40 = 51, 50 = 61. These are with the recommended 20-tooth driven gear.

joe_padavano

You don't need to drop the trans to change the drive gear. You do need to remove the driveshaft and trans tailhousing. You may find that it's a lot easier if you support the trans with a jack and remove the crossmember also, but you may be able to work around it without dropping it. If the drive gear has a retaining clip, pulling the clip should allow the gear to come off. If it's a pressed on gear, you may need a long puller.

66luvr

I'm sure Joe missed the Muncie mentioned in your thread title.
Changing this on an automatic transmission, well a 350 turbo transmission is the only one I have experience with, is simple. But I wouldn't try it on a Muncie 4spd in the car. Maybe it can be done but the reverse deal is on the tail shaft and that might complicate things.
On my 400 turbo in the 66 I had a similar problem, I used a ratio adapter to get it corrected, not sure I got it perfect at all speeds, but it is close enough at or around 60 mph which is what speed I used to calculate my need.
https://www.ebay.com/itm/Gear-Ratio-Adapter-to-speed-your-Speedometer-up-20-/132715427281?oid=362017988462

I bought mine from GM, I knew Chevy used them on trucks with 400 turbo transmissions sometimes as I had seen them on one ton dual wheel trucks I had owned. I figured the % at 60mph and my Chevy parts guy ordered one.
I believe from the numbers you posted you need a 22% overdrive. But do your own math.
And if your speedo is on the driver side of the Muncie you may not have room for the adapter.

joe_padavano

DOH! Yeah, I missed that completely. Thanks for catching that.

BSiegPaint

Well, thank you both for the knowledge and insight. It does sound like the gear ratio adapter would be the easiest route to go - thank you for the link! I had searched but not found that particular one, which is better than what I dug up. My speedo is on the passenger side, so there is plenty of room for the adapter.

BSiegPaint

Well, update on the gear ratio adapter... The seller was great, and helped me choose the right adapter, which was a +28% overdrive and it got here within about 3 days. BUT the piece that connects to the driven gear in the tranny is too short, and there isn't an option for a longer one. Install it and it fits fine, but the speedo doesn't work at all. So I got a purple 17-tooth driven gear and installed it, and now I'm just 3-4 mph slow at about any speed over 20. I can live with that. If and when I ever drop the tranny itself, I'll change the drive gear to the proper one. For now, I guess I'm close enough. Thanks again, everyone