Topic: Calibrating Speedometers
in Forum: C3 Driveline Components
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Lifetime MemberLemon 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.
My speedometer was running about 10% high which on an 85 mph Speedo get you peg real quick. 
The gear housing on my C3 was leaking a little so I had them replace it and in about 10 miles my Speedo quit. Took it back to the shop and the plastic gear shaft snapped off. 
Good time to calibrate the Speedo. Found info on how to select the gears. But finding the correct size was difficult I finally found a 44 tooth gear had it put in ran it on the dino and the car was doing 60 with speedo reading 61mph. Good to go. Took it on the road 5 miles later speedo stopped working back to the shop speedo gear shaft snapped off again.
Good time to calibrate the Speedo. Found info on how to select the gears. But finding the correct size was difficult I finally found a 44 tooth gear had it put in ran it on the dino and the car was doing 60 with speedo reading 61mph. Good to go. Took it on the road 5 miles later speedo stopped working back to the shop speedo gear shaft snapped off again. Guess what speedo gear housing come in two sizes one for 34-41 and then for 42-15 tooth gears. 42-45 will fit but when the housing gets a little hot snap goes the shaft. Not a big problem they are only $11 lesson learned. Not happy but no major damage.
Wrongthere are no more 44 tooth gears that I could find in the world. I happened to have purchased the last one.
There is good news and that what I wanted everyone who may have gone through this and it is that Transmissioncenter.net is manufacturing the complete range of speedo gears now that GM quit doing it. There delivery scheduled to start in early spring 2012. They plan on selling them for $11 each just as before. 
So if you have gone through this issue you may want to revisit it in the summer when the gears are again available in large supply.
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Lifetime MemberIndianapolis, IN - USA
Joined: 9/3/2006
Posts: 2430
Vette(s): Silver 1974 Convertible - L82 4 speed
Low miles (was only 8,900 when I bought it in March 2006) - Now 54K miles. Original owner bought it as an investment for son's college expenses.
2008 Jetstream Blue Convertible
Thanks for sharing so that others hopefully don't have to go through the trouble and frustration you have!
I wanted a Corvette my whole life, but I never dreamed of all the wonderful people I would meet because of it!
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Lifetime MemberModerator
Duncanville, TX - USA
Joined: 11/8/2003
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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
The difference in the housings is that the hole for the gear/cable is off-set, to allow for the different diameter/tooth count gears. Incorrect usage will either break the gear, as Gilbert discovered, or will shred the teeth of the gear quickly. One way puts too much pressure on the gears, and the other puts the gears too far apart.
One thing that could be done, if you find yourself in this predicament, is to turn the housing in the tail, until the gear mesh is correct. The hold-down bracket will not fit into the slots for it very well like this, but it will get you down the road with no speedo issues, and you won't have to get another housing, or gear.
This scenario(speedo gear housing differences) can be had on either the 400, or 700 trannys. The 350(Chevy), and 4 speed trannys use the small "bullet" style speedo housings, and those are all the same.
Thanks for the post, Gil!
One thing that could be done, if you find yourself in this predicament, is to turn the housing in the tail, until the gear mesh is correct. The hold-down bracket will not fit into the slots for it very well like this, but it will get you down the road with no speedo issues, and you won't have to get another housing, or gear.
This scenario(speedo gear housing differences) can be had on either the 400, or 700 trannys. The 350(Chevy), and 4 speed trannys use the small "bullet" style speedo housings, and those are all the same.
Thanks for the post, Gil!
Joel Adams
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Lifetime MemberLemon 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.
So it is not so much the shaft size although the smaller gear shaft move a little easier than the larger gear shaft did. that they are off set a little make it work for me as the shear point in both casees was next to the gear. Luckly I do not see any damage to the drive gear. I looked real close. I know all of the other transmissions that take this set up which is why the after market company is making the investment in them. The engineer said he expected my size to be in in a couple of weeks but he wanted to run test on them to make sure they would last. He stated that they tried metal ones and that did not work out good when tested.
in Forum: C3 Driveline Components
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