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Old 07-12-2004, 05:09 PM
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How To Fix Speedometer After Changing Gear Ratio

I went from a 2.73 rear end to a 3.55 my speeometer is way off. is their a way to fix this problem any help would be great thanks
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Old 07-12-2004, 05:46 PM
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I just did a gear change from 2.29 to 3.73's this past Saturday and have since ordered my new speedometer gears to correct the problem.

It also depends on which transmission you have, I am guessing either a TH-350 or TH-400.

The speedometer gears will need to be figured with the tire size you intend on running with the gear ratio you just installed.

I have seen litle information on replacing the speedo gears on those transmissions as I have not run into the need to do so yet, but for the TH-2004R and TH-700R4 transmissions- Bowtie Overdrives has a calculator on their website and also sells all the individual parts and gears needed for this change.

If I find this info I will post it here AND reference it in my Transmission Info sticky post at the top of this forum also.
TCI's speedo gear calculator and information.

TCI's Speedometer gear part numbers and identification info.

Last edited by M&M CUSTOM; 07-12-2004 at 06:09 PM.
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Old 07-12-2004, 08:54 PM
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i have a 350 th my tire size is 27" i would love to know what speedo gear i need? if anyone can help me
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Old 07-12-2004, 09:57 PM
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Got tire diameter & gear ratio? Go here:

www.novaresource.org/speedo.htm
www.novaresource.org/speedo2.htm
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Old 07-13-2004, 07:43 AM
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Quote:
Originally posted by jimfulco
Got tire diameter & gear ratio? Go here:

www.novaresource.org/speedo.htm
www.novaresource.org/speedo2.htm
Those links are informative but takes quite a while to read and figure what you need, I have had those pages sent to me for my information sticky post.
BUT, the links I posted give a simple fill-in-the-blank calculator.

If you have a drive gear with 15 teeth, you would need a driven gear with either 39 or 40 teeth whichever is available for your transmission.

The easiest way to determine what the drive gears' toth count is look at the color and go back onto the TCI website links above and identify that particular color with it's corresponding assigned tooth count.
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Old 07-13-2004, 07:53 AM
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Another option if you strike out with in-tranny speedo drive gear changes is take it to your local speedometer shop. Most medium sized cities have then - truckers have to certify their speedos so there are shops to service them. Look in the yellow pages. They can put on a little axillary gear box that will perfectly calibrate your speedo and will even issue a laminated certification card!
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Old 07-13-2004, 08:44 PM
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Some of the better transmission shops should have them too. Gears that is. I ended up changing the gear and the small housing it rides in. I was a little less scientific with my trial and error method. Luckily the transmission shop guy is a friend of mine and put up with my searching through his gear collection.
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Old 07-19-2004, 11:14 AM
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My local transmission shop had NO resources to even look up the gearset I needed to correct my speedometer in my TH-700R4.
The calculator over at Bowtie Overdrives told me I needed a 15-tooth drive gear, and a 43-tooth driven gear to correct my speedometer with my existing 26 inch diameter tires and my newly installed 3.73 gears, as the currently installed speedo gears say I am moving 85 miles per hour but am actually going 51, WAY OFF, but no surprise.

My local tranny shop couldn't even confirm the EXISTANCE of the 40-45 tooth driven gear housing, let alone get one.

Besides Bowtie Overdrives, another source with calculators and gear sets with every part necessary for the speedometer gear change is TransmissionCenter.net has parts for several different transmissions. These guys even have the gear box adapters willys36 was mentioning.

Another source for speedometer gears is TCI Automotive, and they too- have an online calculator, plus gears for more than just GM transmissions.

Here is TCI's other speedo gear page with part numbers.

And for you 70SPORT, your transmission drive gear should have 17-teeth, and driven gear should have 45-teeth for a correct speedometer using your 3.55 gears and 27 inch tall tire.

Last edited by M&M CUSTOM; 07-19-2004 at 11:19 AM.
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Old 07-19-2004, 11:19 AM
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Just go to a chevy dealership parts dept. They'll tell you what you need and sell it to you on the spot.

Troy

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Old 07-19-2004, 04:13 PM
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They didn't have exactly what I needed and would have had to order a couple of the individual items for more than what BTO wanted even with shipping.
I got mine in about an hour ago and it pretty much went flawlessly, even for my first time opening a transmission to change the speedometer gears. The right tools help quite a bit.

The only real issue I had with the whole thing was the drive gear still had some of the melted cutting process flash that needed trimmed.

I took pictures of the entire procedure for journal purposes which I will eventually get around to.
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Old 07-20-2004, 03:11 PM
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M&M the biggest housing I could find was a 43 housing #1362077. I used a 43 tooth driven gear with my stock 18 tooth drive gear. That put my speedo within 5 mph, I then added a adapter off of a parts truck that I had and that put my speedo dead on. The little gear box was made by stewart warner, 666f-y10, 8974b.

I have the misfortune of living in a small town so the dealer was absolutely no help and there are no speedo shops for a 100 miles or so.

Also my trans guy is kind of a pack rat. He has at least 5 of everything in the shop or out in the yard. If you want I can post a pic of the housing if you are interested.

John

Last edited by propaniac; 07-20-2004 at 03:17 PM.
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Old 07-21-2004, 06:54 AM
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Bowtie Overdrives had exactly what I needed, I had the phenolic (plastic resin) 34-39 housing already in the transmission, but with my tire size and new gears I had just installed-needed the 15 drive and 43 driven gears I had hoped to get the 40-45 housing from my tranny shop on trade for parts or work but he didn't have ANY.

I had originally ordered just the gearset from Bowtie Overdrives, but after finding my tranny guy had nothing in his shop had to redo the order to include the bigger housing.

My tranny guy did however give me the tailshaft housing seal for free though, and it really was a self explanatory procedure even though Bowties' diagram for the clip and drive gear needed a few simple words to fully explain itsself, and the gears put my speedo back to where it should be.
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