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Old 03-04-2009, 11:05 AM   #1 (permalink)
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Speedo outta calibration?

Hello!

Is there a way to calibrate the speedo?
I believe mine is out.

I have a GPS unit, and in the van, the GPS and Speedo read the same.
But in the GEO, the GPS read 3-4kph SLOWER then the GPS...

Speedo = 80kph
GPS = 76/77kph

I bought the car second hand, has 12" tires on it. would 13" tires correct the difference?

Also, how is this affecting my MPG readings?

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Old 03-04-2009, 11:54 AM   #2 (permalink)
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It would be affecting your MPG readings if you don't have an SG or MPGuino, b/c the speedo controls the odometer.

If you want to correct it, you'll have to change tire sizes. The problem is that most vehicles come equipped with a "margin of error" on the gauges.

If you notice that it's CONSTANTLY 3-4 MPH off, you can correct it by moving the needle. If it's a sweeping correction though (1mph at 20, 2mph at 30, 3mph at 40, etc.) You can only correct it by adjusting gearing and etc.
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Old 03-04-2009, 12:39 PM   #3 (permalink)
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Check the accuracy of the odometer. It is entirely possible that the speedo mechanism is off (internal springs or bearings) but it is keeping accurate track of miles traveled, in which case there isn't much for it except to try and clean the speedometer bearings and/or weaken the spring just the right amount (good luck )
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Old 03-04-2009, 06:43 PM   #4 (permalink)
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Quote:
Originally Posted by Christ View Post
If you notice that it's CONSTANTLY 3-4 MPH off, you can correct it by moving the needle.
Does the dash have to be dropped in order to do this?
Or is there an easier way to remove the window and adjust it?
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Old 03-04-2009, 07:11 PM   #5 (permalink)
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Is it just the speedo that is off or is the odo off too?
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Old 03-04-2009, 08:45 PM   #6 (permalink)
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Quote:
Originally Posted by dcb View Post
Is it just the speedo that is off or is the odo off too?
Honestly, I've never tested it... not sure.
If one is off, the other would be too?
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Old 03-04-2009, 09:12 PM   #7 (permalink)
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not necessarily. Speedos are not historically super accurate, but odometers should be pretty accurate.
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Old 04-06-2009, 11:10 PM   #8 (permalink)
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The whole assembly should be within 3% accuracy. I've got 12.5% low on my truck. I know it ain't the 14" wheels, that is the smallest rim they used.
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Old 04-06-2009, 11:18 PM   #9 (permalink)
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You can usually pull the speedo out without removing the dash entirely. There is normally a bezel that you have to remove in front of/around the cluster.

The odometer is only ever off as a result of tire sizing, as it's gear driven from the speedo cable (or electronically driven from a VSS) the speedo is also driven by the cable, but it's driven by transfer of energy from the cable spinning a "fork" around a weight, and the weight is what's connected to the needle. I'm not really sure how to refer to the force at work there, but it's none-the-less pretty interesting when you see how it works.

What actually happens is that the speedo cable's "fork" section spins furiously around a weight that is round, but they never make contact. The "fork" is basically whipping the air, and that causes the weight to move and hold against a very light spring. The fork produces just enough energy via tight tolerances and air friction to keep the weight in a specific area per MPH, which is displayed on the gauge. This is why mechanical speedo's default to 0 (or further).

For VSS and electronic speedos, I believe it's just a signal input to an actuator which is calibrated as "pulse width input = length or distance output = speed display"

IOW - the VSS signal tells the actuator how far to move, and the actuator is calibrated to move a certain distance to display each MPH.
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Old 04-09-2009, 10:12 PM   #10 (permalink)
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Quote:
Originally Posted by Christ View Post
The odometer is only ever off as a result of tire sizing, as it's gear driven from the speedo cable
I hate to say it but my metros odometer, and speedometer are off, if I'm remembering off the top of my head correctly it is short about 1 mile/mph for every 15 miles/mph I go. and I'm pretty sure my tires are factory sizing.

The only thing I can think of that could by some fluke be the cause is that we changed the speedometer cable to one from a scrap geo because the old one was sucking oil up the cable.

Any ideas?

Ben

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