Speedo way off...!

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aengineguy

aengineguy

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Hi to All,

Currently my speedo shows 60, but GPS shows 43. I had a 39 tooth (green) gear which showed about 20 to 25 off. Now I am about 17 off with a 42 tooth (white). Car is 71 Dart with 28" tall tires and 4:10 gears, brand new cable and the gear holder in the trans is clocked correctly. I did the calculations for the actual rolling radius and I would think I would be right on. When I gutted the car from stock a few years ago, there was an unusual small round device inline in the speedo cable and it had a single wire coming off of it going no where. I chucked it along with 500 lbs. of other weight. Was this some kind of ratio adapter? Picture shows it just to the right of the brake lines coming out of the M\C. Any thoughts!!!!!!
Speedo Thingy.jpg
 
That's what I first thought and then I found a calculator that takes in account the compression of the tire under weight, so the actual radius is less than half the 28" tire. It came out closer to 26.8". Regardless, I should have been pretty stinking close with the 39. Possibly my speedo is wacked out!
 
That's a ratio adaptor in your speedo cable. You're off by whatever ratio it is. I would keep the 39 tooth speedo gear and buy a new cable.
 
I have never seen one of those things but if it's a ratio adapter like Brian says it'd make sense the speedo is off. If you don't have it or know where to get one the only solution I'd know would be find a speedometer from a car like yours that didn't use that adapter and it should be calibrated right.
 
I always thought those were cruise control speed sensors.
A ratio adapter wouldn't need a wire for anything, but a cruise control does.
The Wife's 72 Swinger has one that's coming off because we have a new cable for it.
 
fishy68 said:
I have never seen one of those things but if it's a ratio adapter like Brian says it'd make sense the speedo is off. If you don't have it or know where to get one the only solution I'd know would be find a speedometer from a car like yours that didn't use that adapter and it should be calibrated right.
Click to expand...
That is kind of what I am thinking Tracy....Thanks.
 
TrailBeast said:
I always thought those were cruise control speed sensors.
A ratio adapter wouldn't need a wire for anything, but a cruise control does.
The Wife's 72 Swinger has one that's coming off because we have a new cable for it.
Click to expand...
Does the one on your wife's Swinger look like the one in the picture I posted? I have searched for anything that looks like this and cannot find.
 
Had one just like that on my 75 Duster. It may still be in a box somewhere.... I think it sent a signal to one of the service light control modules to remind you to take it in to clean EGR or whatever after so many miles.
 
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