Systems and methods of calibrating an Inertial Measurement Unit (IMU) on a vehicle are disclosed. In some embodiments, a first tilt angle of the IMU is measured with the IMU while the vehicle is stationary. A second tilt angle of the IMU is measured with an inclinometer or tilt sensor while the vehicle is stationary. The orientation of the IMU is corrected based on the first tilt angle and the second tilt angle.