Current conditions for ensuring the implementation of urban mobility plans require high-quality functioning of urban passenger transport systems considering the minimising travel time and increasing travel comfort, as these factors affect passenger fatigue and their productivity during the working day. The study of changes indicators of the public transport system functioning from the gravity function is carried out in this paper, and the results of these studies are given. Based on the surveys of Lviv residents, the distribution of trips by public transport, private transport and taxis was carried out. It is determined that most often city residents use public transport (the share of such trips is 67% of all movements made by transport). An approach has been developed to determine the number of public transport vehicles depending on their capacity through the total number of passenger seats taking into account this distribution. This approach allows determining the required number of vehicles on the route and using different vehicles by their capacity depending on the passenger flow on the route to ensure the comfort of the trip. Based on this, the analysis of the change in the total number of passenger seats from the indicator of the gravity function for vehicles of different capacities is carried out. In addition, an analysis of the regularities of changes in the number of passenger seats for different by capacity vehicles is carried out, and mathematical models of these regularities are developed. It is determined that at low values of indicator of the gravity function, there is a need for large capacity vehicles, while with the increase in this indicator, the demand for small and medium capacity vehicles increases from 34% to 52%. The developed mathematical models can be used for practical justification of the required number of vehicles on the route, taking into account their capacity to ensure the comfortable movement of passengers on the routes of urban passenger transport.