Compressed air can be used as an energy source for brake systems in medium-heavy and heavy-duty commercial vehicles. The moisture in compressed air, which is due to high temperature and humidity, can be eliminated by using an air dryer. In this paper, drying performance data for a cartridge were obtained and used to develop a drying performance program, to predict the moisture and relative humidity in the air tanks of vehicles. The on-load time, off-load time, air flow, duty cycle, humidity and dew point temperature were calculated according to air consumption. The validity of the program was verified, and it was shown to be able to predict humidity changes in the air tank. The air tank capacity was increased from $100{ell}$ to $130{ell}$ to reduce the duty cycle. Therefore, the regeneration rate decreased from 18% to 15%, but the dew point depression temperature (${Delta}T$) remained above $30^{circ}C$. The duty cycle decreased from 50% to 43%, and the total operation time and power consumption of the air compressor were reduced. In conclusion, fuel savings were obtained by changing the parameters to optimize the system.