[JoeC]

I believe the reason for the Speedo adapter was to get correct Speedo reading within federal tolerance. Adapter use was not governed by the gear ratio but by the gear tire trans combo. If you go one tooth up or down on the speedo gear in the trans it is about a 5 mph change. With the Speedo adapter if you go one tooth up or down on the gears in the adapter it is an about a 2.5 mph change. Some tire trans gear combos needed the adapter to get correct Speedo reading. My Yenko Chevelle TH400 4:10 gear has original trans with adapter and the Speedo reads correctly.