Why doesn't the odometer in my logs match my dashboard odometer?

The FMCSA requires that an ELD read the engine odometer, which is separate from the dashboard odometer.

The ELD mandate is very specific about how ELDs should get the odometer reading for e-logs. The engine odometer is based on engine shaft rotations, and is more accurate than a dashboard odometer, is based on wheel shaft rotations that are affected by changes in tire size and pressure.