Weigh-in-motion (WIM) systems for weighing moving vehicles, the systems having the ability to automatically determine and periodically apply calibration factors to WIM scale readings. Auto-calibration may include transferring both WIM and static weight readings for the same vehicle to a database, associating the weight readings, collecting a number of such weight readings, and analyzing the differences between the WIM and static weight readings to calculate WIM scale calibration factors. The calibration factors may be based on vehicle characteristics such as vehicle weight, vehicle class and/or vehicle speed at the WIM scale.