Odometer question?

I tried the search function to answer this question and came up empty. What drives the trip and main odometer in the 1996 Roadmaster wagon? Is it gear driven off the drivetrain? Controlled electronically from the PCM?

I finally figured out why the gas mileage seems so poor on my new (to me) wagon. The trip odometer is consistently reading 28% less than the driven mileage.

The spedometer is also reading 68-69 mph, but GPS consistently indicates 75 mph. So, the spedometer off by about 10%, considerably less of a disparity than for the odometer.