First of all, make sure your antenna has a clear path to the sats and make sure it is mounted securely in position (mounted and doesn't move). The unit will self calibrate over time to compensate for the ant being crooked or backwards.
Next, Clear out the vehicle dynamics memory. Go to Settings, Hardware Info, Calibration Screen (shows speed sense, mileage traveled, etc.). To the right of each of the three readings hit CLEAR. Clear them all out and drive. This make the system relearn the car from scratch. Should be done everytime the unit is installed into a new or different vehicl