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Speedometer Reads Wrong
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samstrader
Posted 2020-12-19 6:40 PM (#606891)
Subject: Speedometer Reads Wrong


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Location: Beaumont TX
I'm trying to figure out why my speedometer reads fast. I'm going 50 and the speedometer says 60.

I have 205 75R 15 radial tires and their circumference is 85.1 inches. My original tires were 6.70 x 15. I still have one and I measured the circumference of the tire and it is 84 3/8 inches.

I thought the tires must be causing the incorrect speedometer reading but circumference of these two tires is very close so it doesn't seem like tires are causing the incorrect speedometer reading.

What else can cause the speedometer to read fast?

Thanks,

Sam
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StillOutThere
Posted 2020-12-19 7:23 PM (#606893 - in reply to #606891)
Subject: Re: Speedometer Reads Wrong



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Location: Under the X in Texas
Incorrect "driven gear" installed where the speedo cable enters the transmission. Pull it out, count the teeth. Ck the parts book to see what should be in there now. Call JC Auto and get the right gear for your need.
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StillOutThere
Posted 2020-12-19 7:24 PM (#606894 - in reply to #606891)
Subject: Re: Speedometer Reads Wrong



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Location: Under the X in Texas
Incorrect "driven gear" installed where the speedo cable enters the transmission. Pull it out, count the teeth. Ck the parts book to see what should be in there now. Call JC Auto and get the right gear for your need.

Or did someone change axle ratios over the last 6+ decades?
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57chizler
Posted 2020-12-20 5:33 PM (#606920 - in reply to #606891)
Subject: RE: Speedometer Reads Wrong



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I would use the odometer, rather than the speedometer, to gauge the accuracy of the unit. IOW, if the odometer is correct (or close to it) over a known distance, then the gearing is correct and the speedo mechanism is faulty.
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1960fury
Posted 2020-12-20 6:36 PM (#606921 - in reply to #606920)
Subject: RE: Speedometer Reads Wrong



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57chizler - 2020-12-20 5:33 PM

I would use the odometer, rather than the speedometer, to gauge the accuracy of the unit. IOW, if the odometer is correct (or close to it) over a known distance, then the gearing is correct and the speedo mechanism is faulty.


Good advice, since you can adjust the speedometer, but not the odometer.
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samstrader
Posted 2020-12-23 9:16 PM (#607028 - in reply to #606891)
Subject: Re: Speedometer Reads Wrong


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Thanks for all of your advice. How do you adjust the speedometer? Is this in the service manual???

My family was the original purchaser of this car in 1955 and my dad parked it in the garage in 1975. It had 72000 miles on it. I don't think anyone would have changed any transmission gears and certainly not the rear end but I will check. I have a record of repairs and don't see any of this.

But I will certainly check the speedometer gear in the transmission and count the teeth and contact JC Auto. I'll look all of this up in my service manual tonight. Thanks

The odometer does not work now but the speedometer works. I wish the odometer worked.
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1960fury
Posted 2020-12-23 10:22 PM (#607036 - in reply to #606891)
Subject: Re: Speedometer Reads Wrong



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I'd repair or replace the odometer and if it reads correct, leave the gears alone and adjust the speedometer. I believe there is a little spring that can be adjustet. Every old speedometer is adjustable as far as I know. From time to time old adjustment tools pop up on Ebay for out of car bench adjustments.
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57chizler
Posted 2020-12-24 12:48 PM (#607054 - in reply to #607036)
Subject: Re: Speedometer Reads Wrong



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Yes, fatigue in the needle return spring can cause the speedo to read high over time. Friction in the needle speed cup can cause it to read low.
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Mopar1
Posted 2020-12-24 3:10 PM (#607064 - in reply to #606891)
Subject: Re: Speedometer Reads Wrong



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A Garmin will give you the accurate speed.....
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samstrader
Posted 2020-12-24 3:14 PM (#607065 - in reply to #606891)
Subject: Re: Speedometer Reads Wrong


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Thanks Sid and John. I have not pulled the speedometer and checked it out and I didn't know they were adjustable. My speedometer reads faster than I am going by about 20%. I'm going 50 and the speedometer reads 60. I have also oiled the speedometer cable but I did not oil that cup wick in the back. It seems like I must have fatigue in the needle return spring because my needle is moving too far. I need to look in the parts manual for this spring and see if I can understand better. But sure didn't know these things were adjustable. I'll also look into pulling the speedometer. How hard is it to pull the speedometer with the dash in the car?
Thanks.
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1960fury
Posted 2020-12-24 3:23 PM (#607066 - in reply to #606891)
Subject: Re: Speedometer Reads Wrong



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1955 and which make? I have no experience with 55/56 but others here surely do.
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sermey
Posted 2020-12-25 7:58 AM (#607092 - in reply to #606891)
Subject: RE: Speedometer Reads Wrong


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I have added an instruction to this problem in my thread - "Make-Up" your Car!  - SERGE -

http://www.forwardlook.net/forums/forums/thread-view.asp?tid=23691&start=295

 



Edited by sermey 2020-12-25 8:33 AM
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1960fury
Posted 2020-12-27 3:53 PM (#607188 - in reply to #606891)
Subject: Re: Speedometer Reads Wrong



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samstrader - 2020-12-27 1:30 PM

Hi Sid, It is a 1955 Plymouth Savoy.

Thanks for the instructions and pictures Serge. I have a much better understanding now. Will pull the speedometer when the weather warms up and see what is going on.

Thanks for everyone's help.


I copied your reply from that other thread You might want to try or practice on another one first? There are 3 55/56 units on Ebay right now.

https://www.ebay.com/itm/1956-Plymouth-Dash-Speedometer-In-Untested-...
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westaus29
Posted 2020-12-27 7:01 PM (#607193 - in reply to #606891)
Subject: Re: Speedometer Reads Wrong


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Put Here Wego on your smart phone, it shows current speed
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1961plymouthfury
Posted 2020-12-28 8:41 PM (#607213 - in reply to #606891)
Subject: RE: Speedometer Reads Wrong


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plug a Garmin in the out let of your car and that will tell you the actual MPH
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samstrader
Posted 2021-01-03 3:31 PM (#607375 - in reply to #606891)
Subject: Re: Speedometer Reads Wrong


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Thanks Jim and Stephanie. I am using my smart phone to see how fast I am going and it really works good. That's how I know the speedometer is wrong. I was worried that I had tires that were too small and that was making the speedometer read wrong and also making the engine over rev because of higher RPM's with small tires. I researched the tires a lot and measured circumferences on my current tires and an original tire I have in the trunk and they are almost the same so I know now that I am not over speeding the engine and not as worried as much. I would still like to fix the speedometer but and worried now about how hard it will be to remove the speedometer from the dash.
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NC Adventurer
Posted 2021-02-01 3:26 PM (#608455 - in reply to #606891)
Subject: Re: Speedometer Reads Wrong


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I've used Powl's to rebuild the speedo on a Porsche 944 I own. They were prompt, very cost effective and the customer service was great. Here's a link to their site: http://www.powls-speedometer.com/
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imopar380
Posted 2021-02-02 9:42 PM (#608488 - in reply to #608455)
Subject: Re: Speedometer Reads Wrong



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Tire size will definitely affect it. On my 1960 Chrysler - when I bought the car it had P205-75R14 and the speedometer was reading about 6% over what the car was doing. 50 MPH on the Garmin was reading 53-54 MPH I now have P235-75R15 - larger wheels and larger diameter tires and the speedo is now reading just slightly low of the actual speed. Now 50 MPH on the speedo is actually 53 MPH on my Garmin.
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samstrader
Posted 2021-02-18 6:26 PM (#609047 - in reply to #606891)
Subject: Re: Speedometer Reads Wrong


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Location: Beaumont TX
thanks Todd for the reference to Powl's. My odometer doesn't work either so when I pull the speedometer, I'll just send it to them and get it fixed right.
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