Any time you change out the rear end gears, you will need to buy a new speedo gear. They color cordnate them so you know which one to get. If your just changing the gauge itself, then you will not have to mess with the speedo gear. Just unplug the speedo cable fromt the back or the old one and plug it in the back of the new one.
Here is a chart to help you determine the correct speedo for your car.
http://ddperformance.com/speedo_gear_chart.htm