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Topic: Speedometer gear?

in Forum: C3 Driveline Components


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Speedometer gear?

Posted: 9/28/14 4:15pm Message 1 of 6
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QualityTime
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Shelby, NC - USA
Joined: 8/24/2014
Posts: 8
Vette(s): 1975 T-Top

My speedometer reads 7 MPH fast @ 60 MPH. While I had the pan off to fix a leaky shift shaft I pulled the speedo gear it is 35 tooth with a 15 tooth drive best I can tell. this is a T400 is there a chart that would show from where it is "35 tooth" where I would need to be to get the speedometer right.

I tried the one that uses tire diameter but just a .25 inch difference goes from 35 to 41 tooth that seems like a shot in the dark.

Thanks Mark




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Re: Speedometer gear?

Posted: 9/29/14 3:41am Message 2 of 6
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Sarge81
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WEST SENECA, NY - USA
Joined: 12/3/2001
Posts: 2379
Vette(s): 1981 Blue Ice Met. w/pearl ghost flames TKO500 5speed. LS1 Swap 3.45 Dana 44.
Here's a site that has a calculator/chart with all the drive and driven gear colors/tooth count. Scroll down to the TH400 section. They sell speedo gears too. Maybe they can help get you dialed in. 





GEN III 5.7L "LS6" Engine swap
TKO500 5 spd.
3.54 Dana 44
'69 "N11" Sidepipes
Borgeson Steering Box
Born 8/1981
Sequence #3975




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Re: Speedometer gear?

Posted: 9/29/14 9:50am Message 3 of 6
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Adams' Apple
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Duncanville, TX - USA
Joined: 11/8/2003
Posts: 20214
Vette(s): #1-1974 L-48 4spd Cp Med Red Metallic/Black deluxe int w/AC/tilt/tele./p/w-p/b/ Am-Fm/map light National/Regional/Chapter NCRS "Top Flight" #2-1985 Bright Red/Carmine Cp.L-98/auto Member: NCRS, NCRS Texas, Corvette Legends of Texas
In general, a one tooth change on a speedo gear will give about a 2-3 mph difference at the gauge. Unless you have changed tire sizes, or the gears in the past, the speedo should be as close as it was when new. They were not 100% accurate then, either.
The Parts manual shows what combos(engine/trans/diff ratio) took what gears. I can look that up this evening if needed, but with the gears you have, my suggestion would be to go to a 37 tooth(red) driven gear. This would slow the speedo down 5-6 mph, approximately. If it slows it too far, then the next gear would be a 36 tooth(white)..good luck finding one of those! Unhappy Not exactly a common gear.
hth



Joel Adams
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"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"

Re: Speedometer gear?

Posted: 9/29/14 10:13pm Message 4 of 6
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tb2k82ce
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Lemon Grove, CA - USA
Joined: 10/17/2007
Posts: 2041
Vette(s): 1982 C3 Collectors Edition 44000 miles, sat in the sun most of its life, My wife purchased it for me for Father's Day in 2007 from her girlfriend that had it for 19 years. It is on the road again. I'm retired but it is now my daily driver.
This is the site I used to purchase gears.  They have all of them.  They even have gone out and re-manufactured some of the gears that were not available. 


if the link does not work google "PATC" and the word "transmission gears".



Re: Speedometer gear?

Posted: 10/5/14 2:37pm Message 5 of 6
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QualityTime
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Shelby, NC - USA
Joined: 8/24/2014
Posts: 8
Vette(s): 1975 T-Top

Thank you all for your help

All the charts I tried pointed to the 35 tooth which is what I had. So I went with the 2 to 3 mph per tooth and got a 38 tooth and now it's very close 1-2 mph fast but I can live with that.


Thanks again

Mark




Re: Speedometer gear?

Posted: 10/6/14 10:13am Message 6 of 6
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Adams' Apple
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Duncanville, TX - USA
Joined: 11/8/2003
Posts: 20214
Vette(s): #1-1974 L-48 4spd Cp Med Red Metallic/Black deluxe int w/AC/tilt/tele./p/w-p/b/ Am-Fm/map light National/Regional/Chapter NCRS "Top Flight" #2-1985 Bright Red/Carmine Cp.L-98/auto Member: NCRS, NCRS Texas, Corvette Legends of Texas
 Put a taller tire on it, and it'll be just right....LOL
It's dang near impossible to get one exact anymore, since the tires we have available are not close enough tot he original sizes to keep the speedo accurate....and they weren't all that accurate to start with. 1-2 mph off ain't bad at all. Thumbs Up


Joel Adams
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(click for Texas-sized view!)             NCRS

"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"

in Forum: C3 Driveline Components


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