You need a DRBIII, but if you have access to one here is the procedure, please note that the revs/mile must be set in both the BCM(odometer) and the ABS/ACM(speedometer).

http://colorado4wheel.com/content/drb_KJ_speedo.html

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Will this work with a gear change also?

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Gear change does not affect the revs per mile. The revs/mile are read after the gearing, not before. Only tire size affects the speedometer.

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http://www.Colorado4Wheel.com
"Its not about what you can DO with your Jeep, its about where you can GO with your Jeep."
Knowledgeable - But Caustic