Author Topic: Speedometer Issues - 160 MPH vs Std 100 MPH  (Read 7054 times)

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Offline soupy28

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Speedometer Issues - 160 MPH vs Std 100 MPH
« on: June 06, 2006, 10:00:09 PM »
My new TA has a 160 MPH Speedometer installed and it is way, way off. I had my GPS with me over the weekend and the speed is off 20 to 40 MPH depending on the speed. When I was doing 70 on the highway, the speedometer read 110. While it may look impressive, I would rather know how fast I am really going. Not only that, the odometer is also running to quickly. I was going to do some research on the Internet, but thought I would stop here first. Do I need a differnet cable? Is there some type of adapter that mounts at the transmission? Any help would be appreciated. My car is a 78 Black SE with the L80 Engine and an Auto tranny.
Dave Szczublewski
"Soupy"
Weston, FL

What we're dealin' with here, is a complete lack of respect for the law.


Offline ta78w72

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Speedometer Issues - 160 MPH vs Std 100 MPH
« Reply #1 on: June 07, 2006, 12:10:23 AM »
It's not the cable but the gear.  I've never changed one so I'm not sure how to do it....but it has nothing to do with the cable....The gear is on the transmission end.

Offline whwright

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Speedometer Issues - 160 MPH vs Std 100 MPH
« Reply #2 on: June 07, 2006, 12:21:09 AM »
You can get information on speedometer gears here:

http://www.tciauto.com/components/speedometer_gears.htm

The "driven gear" is fairly easy to change, but apparently you sometimes have to change the carrier that it goes in as well.  (That hasn't happened to me yet.)

The "drive gear" is more trouble, and it can be a real bear to figure out exactly what you have.  In my opinion, you might as well just bite the bullet and buy both gears.

To replace the "drive gear" you pull the tailhousing off the transmission.  I've only done it to one Turbo 400, in which case the gear pulls off the shaft while you carefully catch the little spring that holds it in place.  I suppose it might be possible to get the gear out without pulling the tailhousing, but I haven't been able to make it happen (yet).  If the transmission has any age on it at all, the tailhousing gasket will tear as you pull it apart, so you'll need a new one.

Good luck!

Wayne.
1978 Trans Am
1975 Grand Prix
1971 Chevy C20
1972 ...uhem... mustang

Offline Hitman

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Speedometer Issues - 160 MPH vs Std 100 MPH
« Reply #3 on: June 07, 2006, 12:40:27 AM »
I would not go changing the Drive Gear just yet.  You might be able to get away with changing the speedometer gear only and not the drive gear.  They had alot of different combinations even in the same year of car.  For example, you might have a pink speedometer gear with 24 teethe and a drive gear with 9 teeth, or a green speedo gear with 42 teeth and a drive gear with 18 teeth (and yes, I am just making up those numbers because I do not know all of them off the top of my head).  So, you might just do alot of testing with the speedo gears before you go changing the drive gear at all.  The gears them selves are only about $8 to $12 each so it might take a little bit of money to get it right, but you can also retuirn the gears that do not work for you.  
Changing the speedo gear is eash on the Turbo 350 tranny.  Follow the speedo cable down to the drivers side of the transmission and un screw iot from the case.  Once that is off of the unit, there is one bolt that holds a clip which holds the unit in place.  Remove that bolt and clip and you can pull the unit and speedo gear out of the transmission case.  You will loose a little bit of tranny fluid (and I do mean a little, like 20 to 30 drops or so).  You can then swap out the speedo gear, and re-install it back into the case with the clip and bolt.  Then hook back up your speedo vcable and take it for a test run.  I found that it is easier to have a car either follow you or lead you to pace the car for an accurate reading.  I say that because I have never really used a GPS to calculate my speed.  And if that gear does not work right, return it, get the next one and try again.  Maybe over this whole process you might have to add 1/4 quart of tranny fluid but that is it.  Each change out takes about 15 minutes to do.  If that does not work, then yes, you can remove the four bolts that hold in your transmission tail housing, remove it fron the tranny mount and replace the drive gear as well if you can find someone who knows how to decipher the right combination.  Like I said before, even in 1978 with the Olds 403 and 350 tranny, there was like 6 or so combinations that they used in the same year.
Brett Campbell
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1978 Trans Am

Offline whwright

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Speedometer Issues - 160 MPH vs Std 100 MPH
« Reply #4 on: June 07, 2006, 12:50:26 AM »
Thanks for setting me straight, Hitman.
1978 Trans Am
1975 Grand Prix
1971 Chevy C20
1972 ...uhem... mustang

Offline Fire Am

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Speedometer Issues - 160 MPH vs Std 100 MPH
« Reply #5 on: June 07, 2006, 09:43:26 AM »
Just a side note, I swapped a 160 speedo in my 77, and its dead on with my GPS, no gear changes at all.  It is a 4 speed though, if that makes any difference. :wink:
Mike

Offline Eagle 1

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Speedometer Issues - 160 MPH vs Std 100 MPH
« Reply #6 on: June 07, 2006, 10:10:12 AM »
Most transmission shops can tell you what color gear you will need based on the tires size,  what rear end gears, and trans you have.
" He done good didnt he Fred?"
"I'm in pursuit of a black Trans Am. He's all mine so stay outta the way."

Offline Hitman

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Speedometer Issues - 160 MPH vs Std 100 MPH
« Reply #7 on: June 07, 2006, 02:53:48 PM »
If anyone needs to calculate what they need, here is how to do it:

(# of drive gear teeth) x (Axle ratio) x (Tire revolutions per mile)
---------------------------------------------------------------------------
                                           1000

Tire revolutions per mile =

                                      63360
-------------------------------------------------------------------
(Tire Outside Diameter in Inches) x 3.14159

Just plug in the number of teeth on your drive gear, multiplied by the axle ratio, multiplied by the tire revolutions per mile, then devide that all by 1000.
Brett Campbell
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1978 Trans Am

Offline ta78w72

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Speedometer Issues - 160 MPH vs Std 100 MPH
« Reply #8 on: June 07, 2006, 04:25:13 PM »
Geez, now you expect us to do math?  Next you'll be quoting bayes theorem of subjective probability.  And I thought it was hard enough dealing with Metric and Standard!

Offline Hitman

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Speedometer Issues - 160 MPH vs Std 100 MPH
« Reply #9 on: June 07, 2006, 06:28:45 PM »
Well I believe in Occam's Razor Myself.... The simplest asnwer is the right one.  That one was simple... you should see the math that I do forfiguring the coefficient of drag on a car.
Brett Campbell
"Hitman"
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1978 Trans Am

Offline soupy28

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Speedometer Issues - 160 MPH vs Std 100 MPH
« Reply #10 on: June 07, 2006, 06:59:44 PM »
Greetings one and all and thanks a lot for all the great information. I found a place to get the gears, so I will have my work cut out for me on Saturday morning...
Dave Szczublewski
"Soupy"
Weston, FL

What we're dealin' with here, is a complete lack of respect for the law.