continued speedometer troubles. do i need it recalibrated?

SonOfaTomP

SonOfaTomP

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so in my infinite wisdom, i broke the linkage from the speedometer cable that splits to the cruise control when swapping to a 125 mph speedo. i had this fixed at a local mechanioc. now even with this fixed the speedometer doesn't show the correct speed and is about 10 mph off. i'm out of options here. what would bbe my best course of action? take it to be recalibrated? another new speedometer cable? thx
 
Try your old one again first to rile out any problems on the car side. The cables wouldn't cause a wrong reading but it would suck to pay for someone to calibrate it to find out it was for nothing.
 
Mikes5thAve said:
Try your old one again first to rile out any problems on the car side. The cables wouldn't cause a wrong reading but it would suck to pay for someone to calibrate it to find out it was for nothing.
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would it be possible for something in the speedo itself to break?
 
probably, but if it’s working/moving smoothly aside the retarded speed reading, I wouldn’t think so. Id agree with Mike to try the old one again and compare operation and go from there.
I had a 2.26ratio in the rear with a 999trans, and just swapped no issue. I have the old trans out of the car, I can tell you what speedo gear is in the trans in the morning.
 
Did the police cars have cruise control? I wonder if that has something to do with it. That and the speedo gears being different. The police car was an 83 and your car is an 88? I bet something is different between them. I have an 83 and an 88 parts book, I'll start digging
 
1988:
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1983:
20240321_091526.jpg20240321_091802.jpg20240321_091723.jpg20240321_091741.jpg
 
police cars had no cruise control. thats why i at first asked if i needed a new speedo cable for this thing to work, but people said they put 125 speedos in Fifth Aves and it just worked. aye caramba
 
SonOfaTomP said:
police cars had no cruise control. thats why i at first asked if i needed a new speedo cable for this thing to work, but people said they put 125 speedos in Fifth Aves and it just worked. aye caramba
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Probably a different number of teeth on the speedo gear
 
It’s still in the fifth Ave with the factory speedo cable/cruise control and speedo gear. No issues for me.

I’m up and about, will check the speedometer gear out of the trans in a little bit.

Edit: I also never took my AHB speedo apart either. I took my civilian one apart, barely got it back together and decided to leave the AHB one alone. Nor am I the first person to do this. We’ll figure it out, there’s a good hive mind at work here.
 
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It gets you thinking about how they do work though. Not every AHB car had a 2.9x gear in the rear end, some had 2.7x (like the AHB rear end I got from @brotherGood
) it wouldn’t surprise me if some had even higher or lower gearing either…so were they all being built by the factory with the correct/different speedometer gearing in the trans for every AHB/fleet car order that left the factory? How does that certified speedo really work? Are they different if they came from an AHB or Fleet car depending on the original cars factory gearing?

Idk…
 
There's no difference in how the speedometer works. The cable passes through cruise and it was an option on AHB.

The speedometer gear at the transmission is totally dependent on what the rear axle ratio is.

If something is binding with the cable that could affect the speedometer but thats usually when they're jumpy. Chances are good it's something with the speedometer itself but I'd still try the old one again first to rule out any other problems.

Nothing changed by year that would affect the speedometer. One from a 77 will work in an 89. The only difference is they did change the speedo gear housing type at the transmission in the late 70s and that affects the cable and gear type. The older ones the cable screwed onto the housing and the gears had a pin the later ones both the cable and gear slip in.
 
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I got my Speedo from a municipal employee in, Addison Illinois.
He thought it would be a cool souvenir so, he grabbed a couple of them when they retired their M bodies. He couldn’t tell me if they were Diplomats or Gran Fury let alone what the diff gear was.
 
could just be this shop didn't fix the cable right. i'll try my 85 speedo and see what it does. this all just stems from me pulling on the cluster too hard when the plastic piece on the back didn't want to come off.
 
This is my thought. Could be the speedo was 10 mph off in the car it came out of. The only thing that would cause it to be off (provided the car has the correct speedi drive pinion) is the speedo itself.

What I mean is, if the 85 speedo was correct, and the 125 isn't, the only thing that changed is the speedo. You could send it in to be recalbrated ($$$), or change the pinion for one with 2 teeth more or less (depending on if the error is faster or slower), which would get it to read closer to actual speed.

Even certified speedos aren't dead nuts. The reading will change from when the tires are new to when they're worn out due to diameter change.
 
Previous speedogear was a 23 tooth, think it’s a 22 tooth gear in now with the current trans (light translucent blue colored gear). Haven’t really noticed any difference particularly.

image.jpg
 
i'll ask my Father who was the last to drive the old car it came out of. other than that i most likely messed up the speedo when changing the mileage to read accurate to the car it's going into. this faulty speedo came out of a 2bbl AHB that had been sitting for years. i'm getting used to the faulty reading now. 35 is 50. i have taken another 125mph speedo out of another car, an 88 4bbl car. we will see how that one goes. careful not to mess with anything. i'll have another shop check the speedo cables just in case. i'm not even going to bother messing with the odometer thanks for all the helpful tips and wish me luck!
 
SonOfaTomP said:
i'll ask my Father who was the last to drive the old car it came out of. other than that i most likely messed up the speedo when changing the mileage to read accurate to the car it's going into. this faulty speedo came out of a 2bbl AHB that had been sitting for years. i'm getting used to the faulty reading now. 35 is 50. i have taken another 125mph speedo out of another car, an 88 4bbl car. we will see how that one goes. careful not to mess with anything. i'll have another shop check the speedo cables just in case. i'm not even going to bother messing with the odometer thanks for all the helpful tips and wish me luck!
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Take your time, don’t pry or force anything too hard.
It should work out just fine. Unless you put little wheels and tires on the car or changed your rear end gear ratio between pulling the civilian speedo and installing your AHB speedo, it should work and read without any issue.
 
the speedometer saga has come to a happy ending. took another 125mph speedo out of an 88 AHB and with a lot of pushing, enough to break one of the bolt clips, IT WORKS! i no longer have to use my phone as a speedo. i believe the other speedo broke either by me dropping it or when i changed the mileage. for now my mileage is 20k off but i don't even care anymore
 
Mikes5thAve said:
There's no difference in how the speedometer works. The cable passes through cruise and it was an option on AHB.

The speedometer gear at the transmission is totally dependent on what the rear axle ratio is.

If something is binding with the cable that could affect the speedometer but thats usually when they're jumpy. Chances are good it's something with the speedometer itself but I'd still try the old one again first to rule out any other problems.

Nothing changed by year that would affect the speedometer. One from a 77 will work in an 89. The only difference is they did change the speedo gear housing type at the transmission in the late 70s and that affects the cable and gear type. The older ones the cable screwed onto the housing and the gears had a pin the later ones both the cable and gear slip in.
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Correct correct correct. Cruise was an option on AHB, the cable wouldn't effect the speed indicated (unless MAYBE it's not properly lubed and dragging but unlikely), and the adapter gear with certain number of teeth would effect speed.

I guess you're sure it's not reading properly. Seems strange coincidence. I have found M body speedometers to be VERY reliable, unlike F bodies.
 
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