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Old 08-01-2006, 10:02 PM
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speedo

Why would a speedometer show the correct speed when travelling a slower speeds but show a slower than actual speed (about 10 mph) at highway speed?

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Old 08-01-2006, 10:11 PM
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They are supposed to be linear in action..if the speedo head is worn the bearings may be about shot and putting a drag on it causing that.. See most of the older speedo's are like a motor in reverse..the cable drives a magnetic armature which then by the current generated as it turns causes the needle to move..even maybe the little springs in there could be worn or something..Generally this is an off to the speedo shop deal as some of the parts may be hard to come by..

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Old 08-01-2006, 10:35 PM
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Mechanical speedos are inherently inaccurate at higher speeds. ALL speedos have some margin of error. They are typically calibrated to be accurate around 45 mph. They will be a little off above and below that speed. Most factory speedos are off more the faster you get, up to 20% off at high speeds (90-100). Typically when a speedo is registering 120 mph the car is really moving between 100-110. If you're off more than 15% you might want to recalibrate it. Or change tire sizes. Go to www.4lo.com and check out the gearing calculators. One change in tire size will make a 2-5 mph difference in the speedo, depending on the size to begin with.

Electronic speedos are much more accurate, but may also be off a little. +/- 10% is considered accurate for a stnadard car. The old 60s and 70s police certified mechanical speedos were expensive and still within +/-5% accuracy.
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Old 08-02-2006, 08:05 AM
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Thanks for the info. That is what this one is out ...shows 10 mph slower at 60 mph. I will learn to live with it for now.
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Old 08-03-2006, 01:25 AM
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10 mph off at 60 is a bit more than normal, usually only 2-3 mph off by then. Look at the www.4lo.com calculators. you can fix that small error, or at least get closer, by changing tire size. Might not be something you want to do now, depending on condition of tires, but at least you'll know what tire size will bring you back on for later.
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Old 08-11-2006, 04:04 PM
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Speedo way off

What year is your car/speedo. My 41 was way off.
They tell me the magnets can lose their charge eventually. Makes sense. I had mine reconditioned ($75) and it picked up about 10 miles an hour. Then I changed out the gears ($20) to adjust the rest of it. Obviously this should be one of the last things you do, after you have the tranny/rear end/wheels and tires you are going to run on it.
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Old 08-11-2006, 09:52 PM
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[QUOTE=oldguy829]What year is your car/speedo.
QUOTE]
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The car is a '28.
I knon't know how old the speedo. I think it is only a year or two old. I am in the process of purchasing the car however the off reading of the speedo is something the current owner has told me about and doesn't know why at hwy speeds it would not show correctly but at in town driving it appears accurate. I am guess the faster you go the more noticable the amount of the speedo inaccuracy will show.
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Old 08-12-2006, 09:45 AM
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Speedo way off

Your logic is correct. Speedometer error will be a % of speed. 3 MPH off at 30 will be 6 MPH at 60 and 9 MPH off at 90. (maybe a little more if the manufacturer has built in some bias) But perfect at 30 and 10 MPH off at 60 is definetly a speedometer defect.
I would do 2 things to start the process. first, find a reliable way to determine the exact amount of error (radar gun, etc) and second, find a reliable way to spin the cable (remember it runs in reverse, counterclockwise) at exactly 500 rpm and 1000 rpm. The speedo should be marked, but are often: 1000 rpm equals 60 MPH.
If the error is the same at both places, it's all in the speedo head.
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Old 08-12-2006, 09:50 AM
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The cops might think I am crazy going through their radar traps and getting ticketed 5 or 6 times to check the speedo!!
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Old 08-12-2006, 01:24 PM
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As a kid fresh out of college, I worked in the department that, among other things, determined the gearing for the speedometer/odometer. I can't speak for the present, but, in the fifties, the speedometer/odometer error was what I considered deliberate. Now, neither is ever going to be perfect, so a certain amount of error is always to be expected, but, accepting that, it benefits the manufacturer if both the speedometer and odometer read a bit high. If the speedometer is reading a bit high, the manufacturer is less likely to be harassed by owners who felt their speeding tickets were unwarranted. If the odometer is a bit high, owners appreciate the "extra fuel economy." My '57 would register 1.1 miles as I drove between the "mile" roads in Detroit. That's a 10% improvement in fuel economy right there!
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Old 08-19-2006, 02:39 PM
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Quote:
Originally Posted by Gramps
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The cops might think I am crazy going through their radar traps and getting ticketed 5 or 6 times to check the speedo!!

I have never had an officer refuse to radar check my vehicle. I just stop and tell them what I need. In fact a month ago I changed tires and had it rechecked then.

Older speedometers might have weak magnets that need to be redone if the needle and odometer are to remain in sync. Speedos are calibrated at 60 mph.


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