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Speedometer Reads Wrong Jump to page : 1 Now viewing page 1 [50 messages per page] | View previous thread :: View next thread |
Forward Look Technical Discussions -> General Technical Discussion and Troubleshooting | Message format |
samstrader |
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Extreme Veteran Posts: 443 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 | ||
StillOutThere |
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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. | ||
StillOutThere |
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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? | ||
57chizler |
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Expert Posts: 3776 Location: NorCal | 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. | ||
1960fury |
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Expert 5K+ Posts: 7395 Location: northern germany | 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. | ||
samstrader |
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Extreme Veteran Posts: 443 Location: Beaumont TX | 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. | ||
1960fury |
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Expert 5K+ Posts: 7395 Location: northern germany | 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. | ||
57chizler |
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Expert Posts: 3776 Location: NorCal | 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. | ||
Mopar1 |
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Expert Posts: 3033 Location: N.W. Fla. | A Garmin will give you the accurate speed..... | ||
samstrader |
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Extreme Veteran Posts: 443 Location: Beaumont TX | 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. | ||
1960fury |
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Expert 5K+ Posts: 7395 Location: northern germany | 1955 and which make? I have no experience with 55/56 but others here surely do. | ||
sermey |
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Expert Posts: 1208 Location: SWITZERLAND | 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 | ||
1960fury |
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Expert 5K+ Posts: 7395 Location: northern germany | 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-... | ||
westaus29 |
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Veteran Posts: 148 Location: Mandurah, Western Australia | Put Here Wego on your smart phone, it shows current speed | ||
1961plymouthfury |
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Expert Posts: 2633 Location: Minor Hill, TN | plug a Garmin in the out let of your car and that will tell you the actual MPH | ||
samstrader |
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Extreme Veteran Posts: 443 Location: Beaumont TX | 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. | ||
NC Adventurer |
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Veteran Posts: 148 | 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/ | ||
imopar380 |
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Expert 5K+ Posts: 7206 Location: Victoria, BC, on Vancouver Island, Canada | 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. | ||
samstrader |
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Extreme Veteran Posts: 443 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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