Speedo issues

[Big Ray]

o.k., I installed a factory tach last year in my 79. I damaged the stock speedo trying to clean my gauges.
Fast forward to today, swapped in a 84 unit, after swapping the odometer. The stock speedo was a little off on my speed reading as compared to my GPS app
The 84 is WAYYYY off. GPS says 55, speedo reads 38.
Any ideas short of buying a new one?

 

[bucket]

Did you happen to verify the odometer accuracy on your old one? It may be worth checking your new one too.

One of your speedos is bad, but I won't say it's the new one without checking the odo accuracy. Interstate mile markers are useful for that. If your new speedo is ok, the miles traveled will be incorrect the same percentage as your speed reading.

 

[RecklessWOT]

Well, the speedometer is only as accurate as what it was hooked to. Did you use the same cable etc from your old speedometer? If you used something totally different than what your setup was initially, it will read differently. Even taking a trans and cable from the exact same year truck wouldn't ensure it was accurate, tire size and rear end ratio come into play

 

[bucket]

Well, the speedometer is only as accurate as what it was hooked to. Did you use the same cable etc from your old speedometer? If you used something totally different than what your setup was initially, it will read differently. Even taking a trans and cable from the exact same year truck wouldn't ensure it was accurate, tire size and rear end ratio come into play

The speedometers themselves are all calibrated the same. Any cable-driven GM speedo will read the same if swapped around.

 

[bucket]

The reason I said to verify the accuracy of the odometer is because a speedometer can get way out of calibration as far as the needle is concerned, but the odometer reading is gear driven and cannot get out of calibration. It is possible that the new speedo is fine and the old one was completely wrong (just dumb luck that it worked with whatever drive/driven gear, axle ratio, tire size combo is in use currently). But on the hand, it's possible the old speedo was fine.

 

[Big Ray]

The 79 was off, but nothing like the new one.
I was meditating on it, then it hit me
I have the early style cruise control, with 2 speedo cables.
I'll pull, clean, and lubricate both speedo cables.
My cruise doesn't work, but I know the control unit is trying to. There's a way to test it. I'll take it apart clean and lube it also.
I'll keep ya'll posted. 44 y.o. grease can't be good for it lol

 

[Big Ray]

The reason I said to verify the accuracy of the odometer is because a speedometer can get way out of calibration as far as the needle is concerned, but the odometer reading is gear driven and cannot get out of calibration. It is possible that the new speedo is fine and the old one was completely wrong (just dumb luck that it worked with whatever drive/driven gear, axle ratio, tire size combo is in use currently). But on the hand, it's possible the old speedo was fine.

I bent the aluminum needle on the old one. I tried to pull it off to replace it with another.
It wouldn't come off, so I straightened it best I could, and reinstalled it. It never worked the same again. The old one was 14% off to start with due to the larger tires over stock.


If cleaning the cables doesn't help I'll just stick the old one back in. I've learned to use the tach for a speedo anyways!

 

[RecklessWOT]

The speedometers themselves are all calibrated the same. Any cable-driven GM speedo will read the same if swapped around.

well that's what I was asking. The speedometer might be the same, but if he also swapped the the speedo gear (lets say the cable was all fcuked up and someone decided to just take everything off the donor truck between the dash and transmission "just to be safe") or heck, maybe it's not even the same transmission as before. I know the speedometers work the same, I was basically asking to see if the gauge itself was the ONLY part that was changed, otherwise there are variables.

 

[Big Ray]

no, didn't change anything but the speedo.

 

[justhorns]

If speedo is consistent with the odometer, you could change the gear in the tranny to correct it..

 

[Snoots]

The GM manual says to only lubricate the lower 2/3's of the cable. Otherwise the lubricant will work its way up into the speedo and ruin it.