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Speedometer vs. GPS speed - question

939 Views 8 Replies 7 Participants Last post by  Verle
I have a 700R4 with speedometer gears selected to match my gear ratio and tire size in my '69. When I check it with a GPS it reads almost dead-on perfect when I drive to work in the morning, but it is about 6+ MPH off in the afternoon when I drive home on the same freeway (going north instead of south). Can anybody think of an explanation for this? I've checked a couple of times and it is this way each time.

The gauge indicates 65 or 66 in the morning when the GPS says 65, and the gauge shows about 71 or 72 in the afternoon when the GPS says 65.
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^^^thanks, but it would be the opposite in this case, since it is warmer in the afternoon than in the morning. Anything in the speedo head itself that could be temperature sensitive?
I'll try some more tests with the Chevelle when I get a chance. I can also use a different vehicle and see if I get the same results, so I can narrow down the problem to see if it is GPS-related or vehicle-specific. I'm in San Diego, so the temperature differences are not huge. Maybe 50-55 degrees in the morning when it's cold, 75 in the afternoon.
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