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Discussion Starter · #1 ·
my speedometer reads about 29% too slow right now. in order to avoid changing the speedo drive gear (the one on the trans output shaft), i'm thinking about a speedo gear adapter.

i've heard of a 1.35/1 adapter, but is that a cable speed-up or cable slow-down adapter?

are these still available? how much and where can i get such a thing?
 

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Why not just change the speedo output gear in the tailshaft housing? This is the gear driven by the gear on the tail shaft. The gear on the tail shaft typically remains the same and only the driven gear is changed.

That said, depending on your transmission finding the right speedo gear could be a challenge. Gears for TH350's and TH400's are available in a multitude of sizes (for now) but others may be harder to find. I replaced the gear in my TH350 in my truck after I changed gear ratios and tire size and got the speedo right back on track (it was about 15-20% off).

I don't know much about external speedo gear adapters. Maybe someone else can cover that one. I'd assume they could be set up to ratio it up or down.

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[This message has been edited by Rich-L79 (edited 03-22-2001).]
 

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Ya i'm going to be needing to do this also. I'm going from a 2.73 rear with 225/60/14's to 4.11's with 275/60/15's!



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Rich-L79, the speedometer in my '66 with M-20 is reading low so I know I need to change the output gear. I've been told that finding output gears for the M-20 is difficult, but that they use the same output gears as the TH350 -- which are available. Can you confirm or deny?

Thanks,
Greg
 

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The driven gears for a Muncie can range from 17-25T and they can be used in some T350. You can still get them at the dealer. Do you know what the tooth count on the driven gear you have is?
 

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Those speed-O gear reducers are a piece of crap IMHO.
The one I had did not work as advertised! At first the needle started to bounce and only got worse in a very short period of time.

Take the time and do it right, you'll be glad you did

[This message has been edited by Cardiac (edited 03-23-2001).]
 

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Discussion Starter · #7 ·
ok, i was trying to avoid too much info; but here goes:

last year when i had a 4sp saginaw, i bought a 17 tooth gear (to replace a 21 tooth gear) to increase the speedo reading. funny thing is, it DECREASED the speedo reading by maybe 5% (without bouncing or jumping, very smooth). this 17t gear was a brand new sealed GM part. i never solved that problem because the saginaw broke soon after that. then came the M20. anyway, my 3.31 10-bolt broke last month, so i replaced it with a 2.56 12-bolt. (please don't yell at me, i'm paying the consequences now--no launch)

anyway, the current driven gear has 21 teeth (red). i was too dumb to count the teeth on the drive gear when we rebuilt the M20, but i believe it was white. i know the color code for driven gears, but none of the 20+ web pages i've been to list the colors of the drive gear.

i need a 29% increase in the rotational speed of the speedo cable, so i need like a 16 tooth gear (+31%). since i already had a 17t gear with less than a mile on it, i figgered i'd see if a 17t gear (+24%) would satisfy me. well it did the exact same thing that happened with the saginaw! now i doubt the drive gear in both tranys have the exact same defect, and the 17t gear was brand new (GM, sealed when i got it).

why do i want to use an external adapter? well, i don't want to change the drive gear because:
1. i've been inside that M20 before and i'd rather not go through that again.
2. if i don't like the 2.56 and change it back to 3.31, i'll have to swap the drive gear again (see reason #1).

unless a 'speedo gear reducer' simulates a reduced number of teeth on the driven gear, then i don't need a reducer. what i need is an adapter that will increase the rotational speed of the speedo cable.

so. the question is: does anyone know of a 'speed up' speedo gear adapter? are these still available? how much and where can i get such a thing?
 

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Most muncie's came with an 8T drive gear. If your drive gear is the nylon type you can swap it out and put a 10T drive gear and run a 20T driven gear if your tire dia. is 26".
 

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I really don't know if M20 and TH350 gears are the same, sorry. When I was having problems with my TH350 speedo reading someone pointed me to a really great web site that helped me determine which gear I needed and it worked out just great. Maybe try your post in engine/drivetrain? Or maybe search the site for TH350 and speedo gear posts? If I can locate that transmission/speedo gear web site again I'll post the address. I think it was set up my a TC member.

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Rich-L79 and secndgn, thanks for the information -- the link to Fred Aldrich's site was very helpful. I think I can get my speedo error corrected now.

Greg
 

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Discussion Starter · #12 ·
OK, now that we've answered Greg's question, how about mine?

does anyone know of a 'speed up' speedo gear adapter? are these still available? how much and where can i get such a thing?
 
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