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Old 11-29-2010, 11:23 AM
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89 GMC Speedo Calibration

I have an 89 GMC 1/2 ton, 2-wheel drive pick-up with a 5 speed standard shift transmission. We put small'er tires on it and now the speedo is off by 5 mile an hour. It has an electronic sending unit in the transmission. Anyone know how to recalibrate this system?

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Old 11-30-2010, 02:57 AM
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take to a shop that does speedo work.

it's going to require modifying information in the ECM to fix and 89 is still obd-1 which probably means removing the chip, editing it, and re burning it.

the other option is an electronic translator type of unit that gets installed between the trans and the speedo wiring.. never used one myself.. I've just seen them sold at Speedometer World
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