Indian traffic conditions consist of diverse vehicle categories of different static and dynamic properties, making it a heterogeneous flow. The vehicles' complex and dynamic behaviour can be dealt with by converting them into an equivalent passenger car unit (PCU). This study estimates the dynamic PCU values of vehicles in mixed flow conditions for undivided two-lane mountainous roads in India using the speed-area method. The vehicular dimensions and their operating speeds were considered as the prime factors for PCU estimation. The data collected from the sites located in the mountainous region of Himachal Pradesh was used to calculate the PCU value for different vehicles. When compared to the values provided in the Indian Highway Capacity Manual 2017, the PCU value of some vehicle categories was found to be different from the suggested range in the HCM. A variation in the PCU value was observed at different traffic flow, vehicular composition, and V/C ratio. This dependency of PCU values with these three parameters was analysed, and based on that, a set of mathematical models were developed. These models were further validated by statistical analysis with data collected at another study site with similar characteristics. As Indo-HCM 2017 suggests PCU values for plain terrain; these models can help forecast the vehicles' PCU values in the mountainous regions.