Speedometer wrong
02-26-2011, 04:55 PM
#1
Cruising
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Speedometer wrong
I was given a 1977 Corvette from my mom (original owner) this past Christmas. There are several small things (I hope) wrong with it. She let it set for about 10 years. One of the things wrong is the speedometer. I get it up to 80 miles per hour and my GPS says I am doing 70. There is less difference in slower speeds. Anybody have any ideas on what needs to be checked to fix this--I am very paranoid driving it because of this speedometer problem.
02-26-2011, 05:23 PM
#2
Welcome to the corvette club ..it must be a good car..make sure you go over the car real good..the speedo issue should be an easy one..if you are sure its wrong..I know my gps can give me weird speeds depending on how many satellites I am hooked to..anyhow the first thing to check is if you have the stock tire hieght..or size..if that is ok then I would try a speedo shop but you probably don't have one around you so..there is a formula to use when changing the gear in the trans..I don't have it on me but I will look for it..it may be just a tight cable though..BRB!
02-26-2011, 05:41 PM
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Heel & Toe
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My speedo is wothless . when I changed to a 370 gear from the stock high gear (?). I think 55 shows maybe 80 I just go by tach , but you sure can impress your friends with 0 to 100 speeds , I really need to correct it. just kiddin'. just haven't got around to it.
02-26-2011, 07:34 PM
#5
Team Owner
You need to change the speedo gear in the tranny to one with 13% more teeth on it. Remove that speedo drive gear from the tranny and count the number of teeth on it, then multiply by 1.13 and order a replacement gear with that number of teeth (round off to the nearest whole number). I think it should be about 3 more teeth than what you now have. Good luck. 
02-26-2011, 08:35 PM
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02-26-2011, 08:39 PM
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Welcome to the corvette club ..it must be a good car..make sure you go over the car real good..the speedo issue should be an easy one..if you are sure its wrong..I know my gps can give me weird speeds depending on how many satellites I am hooked to..anyhow the first thing to check is if you have the stock tire hieght..or size..if that is ok then I would try a speedo shop but you probably don't have one around you so..there is a formula to use when changing the gear in the trans..I don't have it on me but I will look for it..it may be just a tight cable though..BRB!
I am sure about the speedometer being wrong. The first time I drove it, after it was given to me--I was going to be really good and not speed. The speed limit was 60, so that is what I was doing and being passed by everything from a garbage truck, cement truck, minivan, and 18 wheelers----that is what made me suspicious!!! I was only doing 50 mph on the bypass!!
Last edited by cshelldiver; 02-26-2011 at 09:00 PM. Reason: left out phrase
02-26-2011, 08:41 PM
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After reading this a couple of times, I finally figured out what at speedo is!!! Short for speedometer--duh!!! That's okay, I will learn!!!!
02-26-2011, 08:42 PM
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You need to change the speedo gear in the tranny to one with 13% more teeth on it. Remove that speedo drive gear from the tranny and count the number of teeth on it, then multiply by 1.13 and order a replacement gear with that number of teeth (round off to the nearest whole number). I think it should be about 3 more teeth than what you now have. Good luck.
Last edited by cshelldiver; 02-26-2011 at 08:43 PM. Reason: left out word
02-26-2011, 09:15 PM
#11
Safety Car
You need to change the speedo gear in the tranny to one with 13% more teeth on it. Remove that speedo drive gear from the tranny and count the number of teeth on it, then multiply by 1.13 and order a replacement gear with that number of teeth (round off to the nearest whole number). I think it should be about 3 more teeth than what you now have. Good luck.
yes, do this.. lots cheaper than dropping a size or 2 on your tires.. that will cost you 500$, that gear described above is like 8 bucks...
02-27-2011, 10:30 AM
#12
Race Director
just for due diligence its probably worth comparing the circumference of the stock tires to the ones you have to see how much of a difference you have.
02-27-2011, 10:41 AM
#13
Le Mans Master
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Speaking of tires, if the tires on there now are the ones the car was sitting for 10 years on, you should replace them before you try doing 80 mph again. They may look good, but they can fail at any time.
02-28-2011, 08:49 AM
#14
Melting Slicks
Agree with the bkbroiler post. Read the PSA sticky at the top of this forum...kinda spooky, I remember driving 100+ in my '74 911s with dry rotted and cracking yokohamas... not wise.
