For the best mileage possible, you want to run the maximum pressure labeled on the side of the tire. If this is too harsh or uncomfortable, feel free to deflate the tire as you see fit, and take the bike for a test ride. For comfort, you want the rear tire to compress slightly, roughly ¼”, when seated on the bicycle saddle. Pressure too low may cause a flat tire. Experiment a bit with higher and lower pressures to find the one that suits your riding style the best.