To study the feasibility of fleet CO2 regulations, government agencies have been simulating the entire vehicle fleet with a large number of powertrain technology combinations. The modelling is based on component data from benchmarking or suppliers. One difficulty with this ‘bottom up’ approach is that the agencies have a limited understanding of the many requirements and constraints involved, and may therefore come to optimistic conclusions. As many of the technologies under investigation are already being used on individual vehicles today, it is proposed here to take the ‘top down’ view of studying what these technologies actually achieve. The required data was combined from the EPA Test Car List and the DOE Fuel Economy Guide. Powertrain technologies are characterized by estimating statistically a simple physical model of fuel consumption for groups of vehicles sharing the same technology. This was done for naturally aspirated and turbocharge engines with planetary automatic transmissions, the two groups with a large number of applications. For other technologies the number of applications is smaller, and further normalization is required to draw conclusions. Normalized fuel consumption is then a function of a single powertrain matching parameter. Different normalizations were found to be useful: by vehicle work, by displacement per unit distance, and by engine power. For the baseline technology of PFI naturally aspirated engines with 6 speed automatic transmissions, low power/weight vehicles achieve the best (pareto) efficiency of 30% on the Highway test and 20% on the City test. The efficiency pareto then gradually drops as power/weight increases. Usage of Atkinson cycle, turbocharging and downsizing, cylinder deactivation, and 8 speed automatics has made it possible to avoid the degradation with power/weight. For the already efficient low power/weight vehicles, the major fuel consumption improvement has been on the City test through the use of continuously variable transmissions (CVT). So while there are multiple alternative approaches to improve the fuel consumption of high power/weight vehicles, the options are limited for the already efficient low power/weight vehicles.