Keywords: Regulation overview and future trends, powertrain efficiency enhancement, vehicle energy demand reduction technologies, efficient A/C systems using refrigerants with low global warming potential (GWP) Abstract: This paper compares the economic and climate performance of existing direct expansion motor vehicle air conditioners (DX-MACs) using hydrofluorocarbon (HFC)-134a (global warming potential -- GWP=1300), with secondary-loop MACs (SL-MACs) using hydrofluoroolefin (HFO)-1234yf (GWP<1) and HFC-152a (GWP=138), which both satisfy the EU and Japan f-gas regulations requiring GWP<150 and are listed as acceptable by the US Environmental Protection Agency. The paper includes a part-by-part system manufacturing cost comparison and itemized ownership cost comparison taking into account fuel savings and reduced maintenance. The paper is timely because the Kigali Amendment to the Montreal Protocol on Substances that Deplete the Ozone Layer now requires both developed and developing countries to phase-down the production and consumption of HFCs and at the same time increases in energy efficiency. The developed country transition to HFO-1234yf) is well underway with the EU already requiring refrigerants with global warming potential (GWP) less than 150, Japan requiring GWP<150 after 2023, and the US Environmental Protection Agency offering credits toward fuel efficiency standards for low GWP refrigerants. SL-MAC technology uses a refrigerant contained in the engine compartment to chill coolant that is circulated in the passenger compartment. The design uses a smaller refrigerant charge with fewer connections, which reduces refrigerant leak rates and lowers the frequency and cost of service. The thermal coolant ballast saves energy by turning off the compressor during acceleration and engaging the compressor during deceleration, with inertia converted to cold without carbon emissions. The thermal ballast prolongs the comfort during idle stop.