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Speedometer vs. GPS speed - question

937 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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Brian narrows it to its probable origin(s) in the speedometer assembly!! :hurray:

And, if the cable is the culprit in the discrepancy as stated, with the (afternoon/warm) variance with the GPS being a higher mph, a new
and different speedo trans gear is needed to get in better calibration.

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