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Speedometer vs. GPS speed - question

942 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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Temperature decrease causing the tires pressure to drop resulting in a slighly smalller diameter?

If you install shorter tires your speedo reads higher in relation to your actual road speed. If the GPS is 100% accurate this would seem to make sense.

Doppler effect is my backup theory although doubtful.... :)
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