Weigh-in-motion (WIM) sensors are being used by state agencies for commercial vehicle weight enforcement applications that require a high level of accuracy for determining individual vehicle weights. Existing methods of evaluating WIM weight data accuracy have not been effective for detecting small but consistent calibration drifts. This paper proposes the use of a left–right differential metric that is computed as a percentage difference between the left and right wheel weights on the steer axle of Class 9 trucks for WIM sensors that measure and report individual left and right wheel weights. In this paper, the left–right differential is monitored over time by using statistical process control charts to identify when sensors are exhibiting statistically significant variations. The monitoring procedure can be integrated into existing quality control programs. Data from WIM sites in Indiana and California were used to evaluate the effectiveness of this metric.