Rainwater harvesting is an important way of optimizing the usage of water resources and promoting sustainable development. The objective of this article is to evaluate the potential for potable water savings by using rainwater for washing vehicles in petrol stations located in Brasília, Brazil. An investment feasibility analysis was also performed. The study was carried out by using Neptune, a computer programme for rainwater harvesting analysis. Rainfall data from two meteorological stations were considered, as well as different rainwater collecting areas, tank capacities, number of washings, and potable and rainwater demands. As for the rainwater demands, they were considered as a percentage of the potable water demand, varying according to the day of the week. The tank capacities were varied at increments of 1000 L. The investment feasibility analysis was performed for three different cases, taking into account the potential for potable water savings. It was observed that the average potential for potable water savings by using rainwater is 32.7%, but it can vary from 9.2% to 57.2%. The main conclusion obtained from this work is that using rainwater for washing vehicles in petrol stations in Brasília is a feasible investment for most cases as the net present value is positive for an interest rate of 1% per month.