Complex sentence: Stock car racing technology had fallen behind the technology used in vehicles that are mass produced for the general public because NASCAR didn't take advantage of fuel injection when fuel injected vehicles started to almost completely replace carbuetor-powered vehicles during the late 1980s.
Split into simpler sentences: Stock car racing technology had fallen behind the technology used in vehicles that are mass produced for the general public. The reason behind this is that NASCAR didn't take advantage of fuel injection when fuel injected vehicles started to almost completely replace carbuetor-powered vehicles during the late 1980s.