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Speedometer correction

N

ntmmd8r

Active Member
Okay, I'll throw one more out to the masses. My speedo reads 4.5 mph faster at 60 mph then it really is. This was confirmed twice, once with my GPS and then by my friend, who's a local deputy, and his radar unit. I had gotten a chip from Turbo Tweak before I found this out... the reason I'm mentioning this is, one of the questions asked when ordering a custom chip, is if your speedometer is correct or not. I contacted Eric at Turbo Tweak to see if he could do some kind of magic with his computer but he said that it's controlled by the gears inside the governor in the transmission, that he couldn't do anything for me. I know on older cars with a speedo cable you just change the gears out for one with a different count of teeth. Heck, I haven't even looked under the car for this, but I assume that it's new enough it doesn't have a speedo cable. I hate having the speedometer off, even by a hair. Anybody out there have any experience with this? Thanks for all the help, and I hope to meet some of Ya'll at the Trans Am Nationals next month.
Kenny
 
Bottom line is you have a different tire size or rear gear ratio or both, jack it up and count the revolution's on the driveshaft vs one full turn on the tire to determine the gear and go from there...
 
Nope, stock tires and rear gears. GM didn't have very tight tolerances back in the '80's. I'm wanting to know if they make changeable gears at the tranny that you can change out to even things out...just like you used to be able to simply due in the days of the speedo cable.
 
Yes you can change out the gear in the tranny, they come in different colors and have differing amounts of teeth on them. I have a few some where here. Just small plastic gears.
 
That's odd, mine is 4mph off (fast) but i have a 370 gear and a 255 tire, never bothered me enough to go into the trans to correct.
 
Well I guess I give up. I got the car up on my lift and there was no way I could get my large paws in position to remove the "sending unit" or whatever the term is, for the speedometer, without removing the trans. cross member. So I decided before I got deeper into that mess, I'd contact a couple of places on the old inter web that seem to specialize in speedo gears and what not. Both places first told me that my 200-4R transmission should still have a speedo cable since it was a '89. I explained to them that it was a special edition yada, yada, turbo V6. They both told me that if it had an electric output, which it does, that there is no gears to replace to compensate for my variation in my mph. Now these are both places that specialize in this stuff. So I guess I'll just have to live with it. I did check, and it does have the right rear gear ratio, as well as the stock tires. I know for a fact, from 31 years working at GM that all speedos don't read correctly. I especially saw it during about 6 years of running our trucks during the DVT (dynamic vehicle test). They would calibrate the roll booths twice a day during our shift, so I figure what the video screen was showing was the correct value vs. what the trucks speedometer read. While it's on the lift I'm going to hit Ya'll seasoned professionals with one more thing.
 
Now you are getting your feet wet, these car's are more finicky than any woman you EVER met and trust me, i have met some crazy one's....:boxed:
 
You're driven gear is on the VSS in tail shaft and it is a plastic gear and it's shaped like a cone of sorts. I don't know what color(s) are stock for us but it's similar to the later 700R4's and early 4l60 VSS.
 
All the birds went to gears/electrical speedo in 85 or 86, then somewhere in the 4th gens they went to a reluctor wheel and pickup sensor.
 
When I was still working at GM I think the speedo cable went away with the way of ABS, which I remember our first vehicle with all wheel ABS was the GMC Syclone, which I think was '91.
 
Thanks guys.... I finally got my copy of the TTA Supplement shop manual, and it does show a gear. So... all these so called inter web transmission expert shops I called are full of beans. I can't believe people who do things for a living and don't even know what they're talking about.
 
ntmmd8r said:
..... I can't believe people who do things for a living and don't even know what they're talking about.
Click to expand...

Unfortunately there's all kinds of "experts" out there ... but as mentioned easy to change those gears out (once you get to them) ... let us know how things work out and also which color the original gears are that came from the factory just as a reference for us all.
 
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