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Canadian Poncho Homepage -> Canadian Poncho -> 65-70 Full Size Interior Tech Forum -> Speedometer shows wrong
Post Info TOPIC: Speedometer shows wrong
4SPEED427


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Speedometer shows wrong
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So I always suspected the speedometer showed way too low on my car. This afternoon I confirmed it now that I have a tach. I measured the circumference on my tire by rolling the car on flat ground and did the calculation. Because I know my gear ratio it was easy to figure out and I double checked this with the radar unit in the school zone close by. I also went out on the highway and timed 2 miles with a stopwatch. This also confirmed it, my speedometer shows pretty much exactly 20% too low.

So I was doing all the calculations to change the driven gear when it occurred to me I better check the odometer. Back out to our grid roads. Low and behold, my odometer is near perfect, within the parameters of what GM says is acceptable. It actually shows about 2%-3% too much!

So I know my driven gear is pretty much bang on. Has anyone else had an issue like this? I really don't want to pull the cluster but it looks like I will have to change the speedo head or see if someone can repair it.

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1966 Strato Chief 2 door, 427 4 speed, 45,000 original miles 

1966 Grande Parisienne, 396 1 of 23 factory air cars (now converted to a "factory" 4 speed)
seventy2plus2


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The speedo in my Parisienne reads a bit high or low, can't remember which, but the odometer is bang on, verified by about 10 miles on the highway vs the grid roads.

(for those not in Western Canada, when Western Canada was surveyed back in the early 1900's, it was laid out in rectangles - a grid, 1 mile across by 2 miles long. Every 1 mile east/west there's a 66' road allowance, and every 2 miles north/south there's a 66' road allowance. The road allowance is within the mile, not in addition to the mile. If you find a nice straight stretch of road, it's easy to check our old mph odometers.)

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70 2+2 convertible
70 2+2 hardtop
70 Parisienne hardtop

 

 
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4SPEED427


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I could live with 3-4 miles an hour at 60 but 10 is just too much. Plus, knowing that I have it geared correctly helps me figure out where the issue has to be. I may get a speedo head from a wreck and play with it, just to see if I can figure out how to calibrate it. I don't think I have a parts book with cluster parts or I could check what other cars use the same guts. I bet there's lots of other cars that could donate the right parts.

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1966 Strato Chief 2 door, 427 4 speed, 45,000 original miles 

1966 Grande Parisienne, 396 1 of 23 factory air cars (now converted to a "factory" 4 speed)
Lefty


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i just use an app called awesomeHUD on my phone and it works great to check everything.



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Surrey BC

1963 Acadian Beaumont Sport Deluxe
http://www.63acadian.com/
dcneric


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Carl, wonder if the magnets are getting weak after all the years?

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Eric - Regina, SK

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1967 Mustang Convertible

1969 Firebird 350HO Convertible

1969 Camaro X11-V8

1969 2+2 was a 396

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seventy2plus2


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Dumb question, what is the speedo reading when you're stopped?

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norontcan


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Going to need a set of those numbers that people used back in the 70s when switching from mph to kph.

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4SPEED427


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dcneric wrote:

Carl, wonder if the magnets are getting weak after all the years?




After spending a few hours online reading, I'm inclined to think that is what's wrong. Apparently there is a little bit of adjusting that can be done. And also, apparently most speedometers are set that 1000 rpm cable speed is 60 mph, so I may see if I can find a used 66 speedo and play around with it.

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1966 Strato Chief 2 door, 427 4 speed, 45,000 original miles 

1966 Grande Parisienne, 396 1 of 23 factory air cars (now converted to a "factory" 4 speed)
4SPEED427


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seventy2plus2 wrote:

Dumb question, what is the speedo reading when you're stopped?




Not dumb at all. It does zero properly though, good question.

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1966 Strato Chief 2 door, 427 4 speed, 45,000 original miles 

1966 Grande Parisienne, 396 1 of 23 factory air cars (now converted to a "factory" 4 speed)
4SPEED427


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norontcan wrote:

Going to need a set of those numbers that people used back in the 70s when switching from mph to kph.




The funny thing, it's easy to calculate actual speed because right now if you double what it shows, that's almost exactly actual km/h. When it says 25, I'm doing 50 and when it says 50 I'm doing about 101-102.

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1966 Strato Chief 2 door, 427 4 speed, 45,000 original miles 

1966 Grande Parisienne, 396 1 of 23 factory air cars (now converted to a "factory" 4 speed)
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