[Blue Angel]

Found this old thread and wanted to add some info.

Traditional return style fuel systems have a vacuum/boost sensitive bypass regulator. The fuel pump's output is constant volume, and the regulator varies fuel pressure based on manifold vacuum/boost. Excess fuel is returned from the engine bay to the tank.

The first returnless designs eliminated the return line and hardware as a cost savings. In this design, the fuel pressure is always constant and is set by an in tank regulator. From an electrical power consumption point of view, this system has the least efficiency as the pump is always pumping enough fuel at pressure to feed the engine at maximum power.

The newer systems like the one in the Cruze use an intelligent driver to manipulate the fuel pump RPM to vary fuel pressure and flow as required. As far as I know, no pressure regulator is used so every bit of gas pumped by the fuel pump is required by the engine. It wouldn't surprise me if lots of vehicles are using this system by now.

Older systems that were less efficient due to pumping losses also heated the fuel in the tank, leading to increased evaporative emissions.