Author Topic: Speedo Gear Reducer  (Read 2153 times)

mikecio1

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Speedo Gear Reducer
« on: November 03, 2016, 01:29:31 AM »
Hi all, I have recently changed the rear axle configuration from a 3:73 to a more drivable 3:08. The speedometer gear that is in there now is a green 22 tooth. What I need is a 18 tooth Brown how ever with out taking the Muncie (M20) apart to change the drive gear it will just eat up the Brown gear. My transmission guy recommended a external gear reducer to make the speedo read true. Question is where do I get one and what "reduction" do I need? I saw one on eBay
 http://www.ebay.com/itm/AC-90-Degree-Gear-Speedometer-Reducer-Ratio-Adapter-1-0-950-6479236-w-bullet-/122203171020?hash=item1c73e068cc:g:MVAAAOSwcLxYFH3L&vxp=mtr
But I have no idea if it will work or not.
Any help is appreciated
Thanks
Mike

X33RS

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Re: Speedo Gear Reducer
« Reply #1 on: November 03, 2016, 02:47:56 AM »
I buy them from here   https://transmissioncenter.net/?s=speedo+adapter   

Jim is easy to deal with.   He'll ask you to take a reading with a GPS and compare your speedo at 45-55-65 mph and he'll use that to average the percentage needed to change.  They are built on a car by car bases so it takes a few weeks.

Sauron327

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Re: Speedo Gear Reducer
« Reply #2 on: November 03, 2016, 11:23:37 PM »
 Jody's Transmissions can supply the speedo gear based on ratios and tire sizes. No GPS required. He just did an M22Z for my customer. Very helpful and professional.

mikecio1

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Re: Speedo Gear Reducer
« Reply #3 on: November 04, 2016, 12:09:50 AM »
Does Jody's Transmissions have a webs site please? However I do not want a speedo gear, I need an adapter that will make my speedo right. Changing the speedo gear will not help unless I drop the transmission tear it apart and change the internal gear. Which I am trying to avoid. Thank you both for the information!

Sauron327

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Re: Speedo Gear Reducer
« Reply #4 on: November 04, 2016, 12:44:40 AM »

bcmiller

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Re: Speedo Gear Reducer
« Reply #5 on: November 04, 2016, 11:43:08 AM »
You are sure just changing the driven gear won't work?

What is your combination?
Bryon / 1968 Camaro SS 396 - now 468 ci, M21, 12 bolt
Looking for 68 Camaro with body number NOR 181016

Kelley W King

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Re: Speedo Gear Reducer
« Reply #6 on: November 04, 2016, 12:09:11 PM »
IMO changing the drive gear will get you close enough for a muscle car. If you think in the OLD days police speedometers were marked certified. That kind of says the  OEM units were not that accurate. Before I Phone GPS I used to get a friend with a newer car to clock me. If you are really serious most I phones will tell you more than you need. Instead of updating my GPS on my boat I just use the phone app.
69 Z28 RS Scuncio Hi Performance
69 SS L78
67 SS Chevelle
64 Corvette
66 GTO Tiger Gold
77 Trans Am Special Edition

X33RS

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Re: Speedo Gear Reducer
« Reply #7 on: November 04, 2016, 12:56:07 PM »
You'll need a way to determine just how far it's off.  Doing the math works.  The GPS is just a suggestion and the easiest way to do it.  If you have a phone then use that.  I personally don't have smart phones.  Having someone follow sometimes isn't the best way.  Not only hard to correlate but you also don't know how far off the speedo is you're comparing to.  The reason Jim wants a "range" from 45-55-65 is for accuracy reasons.  You can have a speedo that reads closer in the lower mph ranges and be less accurate in the upper mph ranges.  Jim takes this into consideration when building your reducer and corrects that discrepancy giving you a very accurate speedo throughout the range.

  The problem with some of these muncies are the drive gears.  Not a lot available and some aren't currently made.  A certain drive gear only works with a certain range of driven gears so you just can't stick any driven gear in there you want, even if the math calls out a specific tooth count.  Going too far eventually eats the plastic teeth.
   As an example, I have a 66 M21 that is this way.  It was optioned in a vette with either 4.11's or 4.56's.  I'm running it in a 3.55 geared car currently.  I simply cannot remove enough teeth on the driven gear to speed up the speedometer enough.  It chews up the gear within 10 miles.   The correct drive gear isn't produced so the only option is to run the correct driven gear that lives with this specific drive gear, which slows the speedo, then run an external gear reducer to correct the reading.
   And yes, I strive to make all my speedometers accurate.  MPH and odometer accuracy is important to me.  If I were going to live with it being inaccurate, I may as well just disconnect it because it would be no good to me otherwise.


If going with a new trans like an M22Z that are basically modified muncies, Autogear can simply provide the correct drive and driven gears that they make for that trans if you provide the gear ratio and tire height since these transmissions use the larger diameter output shaft.  More of those drive gears are available.  Makes things easy.  I do the TKO swaps here and it's the same deal.  Unfortunately some of the drive gears for the old muncies and small output shafts just haven't been available.
« Last Edit: November 04, 2016, 01:22:08 PM by X33RS »

mikecio1

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Re: Speedo Gear Reducer
« Reply #8 on: November 04, 2016, 01:31:02 PM »
The Steel Drive Gear is a 1:76 Small, that is why the 22 tooth Green worked with it. When I put the Brown gear in, it chewed it up in about 300 miles. I knew it was going bad because of Speedo bounce. Finally it stopped working. I called Bruce at Manchester Muncies, he is always very helpful. He stated that if I had a 22 tooth grey I would be able to just swap to the Brown. The color of the 22 tooth is a give-away as to what is in there. The 1:76 drive gear works with 22 tooth green and usually used with 3:55 gear and up. He suggests rather then drop the transmission to get an external reducer (or increaser ). Not worth dropping a perfectly working trans and tearing it apart to change a speedo drive gear. So I need to figure how much it is off and find the right "adjuster". The speedo is about 10 mph or so off using a GPS speedo app on my phone against the speedo. I will have friend ride with me so we can take an exact reading rather then me drive, watch the speedo and gps at the same time. Thank you everyone for your experiences, opinions and suggestions.

On a side note, Kelley, I retired after 35 years of Law Enforcement. The "Certified Speedometer" was just as accurate as a normal speedo. It just means they allegedly double checked them at the factory to make sure they were accurate. I think it was just a scam to charge more :P If the weren't they still sent them out, I drove a car ( Gran Fury) that was always about 5mph off back in the 80's. Our fleet manager didn't seem to care. Florida requires that the speedo in a police car be certified every 6 months if you use it to enforce speed. So if your speedo was off by 5mph you knew it and had a certificate, to show that you knew what the correction was to enforce accordingly. Now with the development of electric speedo's they are always pretty much accurate. Police package cars sometimes still say certified. However Fords Expedition and large trucks that are used in LEO applications are Fleet not Police and do not have certified written on the speedo. That is another reason the speedo has to be checked every six months.

bcmiller

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Re: Speedo Gear Reducer
« Reply #9 on: November 04, 2016, 02:16:59 PM »
I am going to assume you have read this thread? Link below.

http://www.camaros.org/forum/index.php?topic=2744.0

If that still doesn't help, I have 3 speedo adapters, and might be able to loan one out for you to try before you purchase a new one.
Bryon / 1968 Camaro SS 396 - now 468 ci, M21, 12 bolt
Looking for 68 Camaro with body number NOR 181016

mikecio1

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Re: Speedo Gear Reducer
« Reply #10 on: November 04, 2016, 07:35:12 PM »
Hi Bryon, Yes I looked at the thread. Good information, the drive gear I have is the "small" one used on the higher tooth driven gears 3:70 and up. I need a Brown driven gear to make the speedo work correctly which needs a 'Large" drive gear. That is why I need an adapter. Unless I want to drop the transmission. I am going from a 3:73 to a 3:08, most of the adapters are designed to work the opposite direction I want it to go. So I just might have to have one made?

bcmiller

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Re: Speedo Gear Reducer
« Reply #11 on: November 04, 2016, 08:18:58 PM »
The three adapters I have are to go the other direction, so they won't help. 

But let me do some thinking.  If I come up with another solution, I will let you know. 
Bryon / 1968 Camaro SS 396 - now 468 ci, M21, 12 bolt
Looking for 68 Camaro with body number NOR 181016

bertfam

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Re: Speedo Gear Reducer
« Reply #12 on: November 04, 2016, 08:55:10 PM »
Give Jim a call. He makes adapters to SPEED UP or SLOW DOWN your speedometer reading.

Ed
Ed "Bert" Bertrand
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mikecio1

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Re: Speedo Gear Reducer
« Reply #13 on: November 04, 2016, 08:58:39 PM »
Thanks Ed, I will call him Monday.

Sauron327

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Re: Speedo Gear Reducer
« Reply #14 on: November 04, 2016, 11:05:09 PM »
Jody's tranny knowledge is not limited to Autogear, his knowledge encompasses all Muncies.