How to swap a 79 speedo into an 80?
10-20-2014, 09:41 PM
#1
How to swap a 79 speedo into an 80?
I would like to swap a 1979 140 mph speedo into my 1980. What would I have to do to get it to register properly? Would it need to be recalibrated or would I just need to change the driven gear in the tranny? I searched and found a lot of threads on this with no definitive answer. Some threads say direct swap, others says you need to have it recalibrated, some say you need a traducer and still others say the driven gear will need to be changed. Anyone know for sure? My 80 has a turbo 400 auto tranny if that makes a difference.
Last edited by kaz1961; 10-20-2014 at 10:48 PM.
10-21-2014, 10:22 AM
#3
Drifting
Should be bolt in- These are mechanical speedos.
Speedos work in cable revolutions per mile.
IE 60 MPH is X number of revolutions.
If the speedo is calibrated to 0-60, 0-140, 0-200 etc shouldn't matter.
They should still be calibrated to X number of revolutions per mile.
We used to swap out the old 85 MPH speedos in GM cars all the time for their "Police" equivalent and never had to re-calibrate or install new trans gears.
Speedos work in cable revolutions per mile.
IE 60 MPH is X number of revolutions.
If the speedo is calibrated to 0-60, 0-140, 0-200 etc shouldn't matter.
They should still be calibrated to X number of revolutions per mile.
We used to swap out the old 85 MPH speedos in GM cars all the time for their "Police" equivalent and never had to re-calibrate or install new trans gears.
10-21-2014, 03:44 PM
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10-21-2014, 04:29 PM
#7
Drifting
10-21-2014, 05:46 PM
#10
Le Mans Master
While the technical knowledge required to pull off a speedo swap is fairly low, actually doing it doesn't qualify as something I'd call fun. On a scale of 5, tho only a 1 skill wise, I'd put the hassle level nearer a 4. Still, IMHO stepping up to a 140 (or 165) MPH one is definitely a worthwhile project. YMMV
10-21-2014, 05:47 PM
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How can that be when an 85MPH max speedo a completely different sweep has than a 140MPH max speedo?Seems to me that a 85max speedo would have to spin slower internally to make the complete sweep to a "pegged" 85mph,where as a 140mph speedo wouldn't.I mean if I was to take the face off my 140 speedo and put an 85 mph on,would'nt I have to run 140 for it to show 85?
Last edited by Ravoll; 10-21-2014 at 05:50 PM.
10-21-2014, 05:51 PM
#12
Le Mans Master
IIRC speedo calibration is weighted magnetically relative to its given MPH range to register whatever speed is being input via the cable.
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Last edited by TheSkunkWorks; 10-21-2014 at 05:54 PM.
10-21-2014, 05:56 PM
#13
Drifting
How can that be when an 85MPH max speedo a completely different sweep has than a 140MPH max speedo?Seems to me that a 85max speedo would have to spin slower internally to make the complete sweep to a "pegged" 85mph,where as a 140mph speedo wouldn't.I mean if I was to take the face off my 140 speedo and put an 85 mph on,would'nt I have to run 140 for it to show 85?
If that were the case, GM would have to list a separate speedo head for every axle/tire combination possible (which they don't).
That is why they have so many speedometer gear options in the trans-
Speedometers are calibrated to the 'face' they have, period.
You are comparing apples to oranges in your example above-
If you put a different face on the same speedo head then yes, it would need to be re-calibrated- But if you are swapping the entire unit, it's all good and it'll all work properly with no calibration required.
10-21-2014, 06:14 PM
#14
Drifting
Nope.
If that were the case, GM would have to list a separate speedo head for every axle/tire combination possible (which they don't).
That is why they have so many speedometer gear options in the trans-
Speedometers are calibrated to the 'face' they have, period.
You are comparing apples to oranges in your example above-
If you put a different face on the same speedo head then yes, it would need to be re-calibrated- But if you are swapping the entire unit, it's all good and it'll all work properly with no calibration required.
If that were the case, GM would have to list a separate speedo head for every axle/tire combination possible (which they don't).
That is why they have so many speedometer gear options in the trans-
Speedometers are calibrated to the 'face' they have, period.
You are comparing apples to oranges in your example above-
If you put a different face on the same speedo head then yes, it would need to be re-calibrated- But if you are swapping the entire unit, it's all good and it'll all work properly with no calibration required.
