Correcting a Speedometer
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Joined: Aug 2004
Posts: 65
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From: Lakeland Florida
Correcting a Speedometer
Puzzled here. The speedometer was off approximately 20% (due to rear gear change). I replaced the 35 tooth gear (behind the speedometer cable inside the automatic transmission). I installed a 45 tooth gear. It made no change at all. How can this be possible?
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,114
From: Crossville TN
Did you check it at different speeds (i.e., 30, 45, 60, 75mph) or just at one speed? It is possible that the problem is in the speedo head and that it is just slipping and giving an erroneous reading. Otherwise, you should have gotten different results with the two gears.
Safety Car
Joined: Sep 2007
Posts: 4,454
Likes: 328
From: Indy Indiana
i agree, not possible to not change when you swap the driven gear...
i fought speedo calibration issues for years, I had a 4.56 and changed to a 2.73.. turns out a driven gear was not available for my drive dear/rear ratio combo so I had to go with a specific speed up factor. i used a gps to figure out my error and calculated a difference and called up these guys..
best 70$ I spent....
http://www.transmissioncenter.net/sp...n_______va.htm
i fought speedo calibration issues for years, I had a 4.56 and changed to a 2.73.. turns out a driven gear was not available for my drive dear/rear ratio combo so I had to go with a specific speed up factor. i used a gps to figure out my error and calculated a difference and called up these guys..
best 70$ I spent....
http://www.transmissioncenter.net/sp...n_______va.htm
Safety Car
Joined: Oct 2004
Posts: 3,921
Likes: 731
From: Youngstown Ohio
Also, you cannot put a 45T gear in the same housing as a 35T originally was......it will fit, but will not engage properly. There are 2 housings for speedo driven gears....1 for lower numbers 33-37T (don't quote me exactly, I'm trying to remember) and 1 for higher 38-45T I believe. The numbers for the teeth are shown on the housing. Both housings fit in the trans, but the right gear must be in the correct size housing to work.
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Joined: Aug 2004
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From: Lakeland Florida
Thanks C3Stroker. It slide into place easy enough. Actually still works "exactly" as before. Do you know where to order the different housing? It has approximately a 4.00 or taller gear. Could easily be 4.11. If it was the housing, would the 45 tooth gear even work in it at all?
Burning Brakes
Joined: Feb 2009
Posts: 780
Likes: 1
From: Shelby NC
BTO has a calculator to tell you what size you need based on ratio and tire size--all this on the web page. They also have the housing but you can get them from GM also--I just got one for 40 up size gear.
Team Owner
Joined: Jul 1999
Posts: 65,492
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From: Orange Park Florida
Believe two things.....GPS/Garmin......
and that those tire size rpm speed sites are RONG.....
you are not as fast as you think, only 1/2 assed.....
seriously, and faster you go, the more inaccurate your speedo is....
OR maybe the errors are more readable was a friend's comment....
I know that my speedo is very accurate at 15-75 mph.....at 80 it's only really 78, at 100 it's really only 94 at that point I ran outta room....
not bothered to check again....
do not trust your speedo to be linear in response.....
and do not trust your tach either....
I did a test cal on my electronic tach...0-1500 rpm was fine....
3000 rpm was over indicating by 300....10% over....
4500, 6000 same thing 300 rpm over indicated....
go figger....I dunno, all I can say is I know the tests were valid....
and that those tire size rpm speed sites are RONG.....
you are not as fast as you think, only 1/2 assed.....
seriously, and faster you go, the more inaccurate your speedo is....
OR maybe the errors are more readable was a friend's comment....
I know that my speedo is very accurate at 15-75 mph.....at 80 it's only really 78, at 100 it's really only 94 at that point I ran outta room....
not bothered to check again....
do not trust your speedo to be linear in response.....
and do not trust your tach either....
I did a test cal on my electronic tach...0-1500 rpm was fine....
3000 rpm was over indicating by 300....10% over....
4500, 6000 same thing 300 rpm over indicated....
go figger....I dunno, all I can say is I know the tests were valid....
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Safety Car
Joined: Oct 2004
Posts: 3,921
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From: Youngstown Ohio
Thanks C3Stroker. It slide into place easy enough. Actually still works "exactly" as before. Do you know where to order the different housing? It has approximately a 4.00 or taller gear. Could easily be 4.11. If it was the housing, would the 45 tooth gear even work in it at all?
Any gear will physically fit in either housing, but one not correctly sized will not always mesh or meet with the drive gear in the trans. The housings space the driven gears at slightly different lengths corresponding to the number of teeth. Also, I believe the housing must be oriented correctly into the trans. I think when it's turned correctly, it fits right in.....the orientation is indicated on the housing.