The major challenge to the large-scale commercialisation of hybrid electric vehicles is their high initial and operating costs, although a mild hybrid is generally cheaper than a full hybrid of the same performance. Hence, the average operating efficiency of the internal combustion engine of a parallel hybrid and the energy saved by its battery modules when running on urban roads have been key research subjects. In this paper, a method for maximising the performance of heavyweight hybrid vehicles is proposed. Because the manipulation of the thresholds of the state of charge window of the battery of a parallel hybrid affects the performance of the vehicle when using the commonly employed electric power-assist strategy, we developed a method for determining the optimal sizes of the vehicle components that afford the most feasible operations using the conventional and modified power-assist strategies. Furthermore, the performances of the vehicle in both modes were analysed for various urban driving conditions representative of those encountered in the USA, Europe, and India. The results showed that energy saving in the battery pack after a typical urban driving cycle was higher in India than in the USA and Europe. The saving in India was determined to be 3.34%, whereas those in the USA and Europe were 3.13% and 0.23%, respectively. The energy savings in the battery pack of the same vehicle under identical conditions were determined using a simple internal resistance battery model and a resistance–capacitance battery model. The performance of a typical parallel mild hybrid capable of operating in the modified power-assist mode was also compared with that of an equivalent internal combustion engine vehicle presently on the Indian market.