The adapter is a gear reduction box that calibrates the speedo to read correctly in the case of rear axle ratios of 4.56 and 4.88. This was required in cases like this. For 4.10 and higher ratios, it is not needed.

So, if your car has a 4.10 or higher ratio, then it is not needed. Most 4.10 ratios will use a 25 tooth driven gear and 1.750" drive gear. 3.73 ratios will use the same drive gear and a 23 tooth driven gear. The driven gears are color coded. 23-t 3.73, black and 25-t 4.10 orange.

Jerry