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Speedo/odo Question.

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A couple weeks ago I finally corrected my speedo gear to adjust for larger tires.  According to my GPS and those  "speed checkers" on the side of the road the speedometer is spot on.  I  thought that the speedometer and the odometer  both run off the same gear/cable.  Maybe it does but if I go by the mile markers on the road AND the GPS my odometer is still off by about 15%


Is there something I'm missing.  I took a little road trip yesterday and I know for a fact that the route is 121 miles from my house to my destintion.  So, round trip odometer should read 242 miles but it only reads 207 miles.  Didn't take any side trips, shortcuts or nuthin'...and my wheels were not off the ground for 35 miles. 

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They do run off the same drive gear. But ... the odometer has a completely mechanical connection all the way from the transfer case through the cable to the number indicator in the cluster. The speedometer head isn't a direct, mechanical connection at the head. The dial needle is mounted on a steel cup that's free to rotate, and has a spring on it. Inside the cup, there's a slightly smaller diameter magnet that is driven by the speedo cable. As the magnet rotates faster inside the cup, it pulls the needle against the spring. There is no mechanical connection between the cable and the needle.


It's actually unusual for an old-style speedometer like this to be 100 percent in agreement with the odometer. As the speedo ages, the spring may lose tension, which allows the needle to rotate easier and to read faster than the actual speed. The only way to get the speedo calibrated to/with the odometer is to take the speedometer head to a speedometer repair shop (if such even exist any more) and have it recalibrated.

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