Reduction in vehicle weight can significantly reduce energy use in human transportation. However, to gauge efficiency, energy use and weight for a particular vehicle should be related to the number of people being transported while currently there is no convenient means to assess this. Here we statistically analyse the weight, energy consumption, carrying capacity and occupancy level for automobiles, buses, high-speed trains and aircraft. Based on the analysis and inspired by the medical body mass index (BMI), we have proposed a vehicle mass index (VMI), defined as [Formula presented], for the first time enables energy efficiency assessment of different transportations on a global scale, where n a weight sensitivity parameter and A the energy efficiency constant of a theoretically weightless vehicle. We show the VMI ranges and conclude the significant vehicle weight reduction windows to achieve their index lower limits. The possible limits for the VMI and the associated A and n values are also assessed. The concept of VMI could form the basis of a worldwide standard, useful in the current drive for a greener economy.