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| Speedo/Odo accuracy question https://www.slantsix.org/forum/viewtopic.php?t=17067 |
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| Author: | Viking [ Sat Apr 15, 2006 8:06 am ] |
| Post subject: | Speedo/Odo accuracy question |
I'm curious - if my odometer is accurate, does that mean my speedometer is also accurate, since they run off the same mechanical cable? I've compared the odometer to measured highway markers, so I'm curious to know if the speedo is telling me the truth or not. The car is a '71 Dart 4dr, 48,000 original miles. I know that these speedo's have a reputation to be notoriously optimistic when reporting your speed - I just want to know if there is a linear relationship between speedo and odo. Thanx! |
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| Author: | SlantSixDan [ Sat Apr 15, 2006 8:51 am ] |
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Generally yes, if the one is accurate the other is accurate. When you run mileage checks, be sure to use ten or more mile markers, not just one at a time. |
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| Author: | emsvitil [ Sat Apr 15, 2006 1:41 pm ] |
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I'm going to disagree with Dan here........... It's been my experience with mechanical systems that they can disagree quite a bit. The odometer is purely mechanical. Get the speedo gearing to match with the tire diameter and rear-end ratio and it can be quite accurate. But the speedometer is magnetic. The needle deflection is based on how fast an internal magnetic is spinning inside the speedometer, the spring tension on the needle, how far the pieces are apart internally...... So unless everything is calibrated perfectly, the speedo and odometer won't agree. Some cars are worse than others. With my 64 valiant, the odometer is within 1%, but the speedometer varies from 3% to 7% high (faster you go the worse it reads high). My toyota truck has about a 2% difference between the two readings. Part of the higher speedo readings is calibration, part is regulations that allow manufacturers to read high (but not low), and part is the manufacturers wanting you to think you're going faster than you are. |
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| Author: | SlantSixDan [ Sat Apr 15, 2006 2:19 pm ] |
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Whoops, I was assuming that the only variable was speedometer pinion vs. final-drive ratio and tire size, not whether the speedo/odo head and cable are in good condition or are faulty. If there is significant wear in the head unit, or a dry bushing, then yes, the speedo can run slow relative to the odometer. |
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| Author: | slantvaliant [ Sat Apr 15, 2006 2:25 pm ] |
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The easiest way to check speedometer accuracy is with a GPS. |
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| Author: | 1966 dart wagon [ Sun Apr 16, 2006 2:44 pm ] |
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Quote: The easiest way to check speedometer accuracy is with a GPS.
yeah this way works good, i did this in my dart cause the speedo only worked at 30mph+ i dont know hy, but it was correct even up to about 50 or so, i hardly went over that, but i did hit 75 one time, no tach but wow she was screaming. 
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| Author: | Viking [ Mon Apr 17, 2006 11:24 am ] |
| Post subject: | Speedo/Odo accuracy |
Thanks folks - you tend to confirm my thoughts, that even with an accurate odo, the speedo can be out to lunch! Appreciate the responses! |
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| Author: | Viking [ Mon Apr 17, 2006 11:24 am ] |
| Post subject: | Speedo/Odo accuracy |
Thanks folks - you tend to confirm my thoughts, that even with an accurate odo, the speedo can be out to lunch! Appreciate the responses! |
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