Speedometer Gear Question

I just went from 2.8:1 gears to some 3.55s and have found (using my GPS) the the speedometer is WAY off. I know I need to change the speedometer gear, but don't know what kind to buy or how to change it.

The car has the Ford C4 3 speed auto transmission, and 215/70r14 tires on it, with newly installed 3.55:1 gears.

What speedo gear do I need and how do I put it in?
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