This paper addresses the question of fuel efficiency bounds for Internal Combustion Engine (ICE) powered vehicles. Such bounds are based on the optimally achievable brake specific fuel consumption (bsfc) of current state of the art engines and the ability to limit the engine operating conditions to the bsfc minimum. Three different vehicle classes are represented by one typical vehicle. Based on these bounds, an efficiency index is defined that measures how close a vehicle power train system comes to achieving this bound. This index is given as a ratio of theoretically achievable fuel consumption to the actually achieved fuel consumption of the vehicle for a speed range that is selected via a weighting function. Our results indicate that even the most efficient power train systems provide a fuel efficiency that can be improved by a factor of approximately 1.5 to 7, depending on the speed profile.