^^ That 3.5 may need to move the accelerator to keep the speed constant- the RPM will not change if the speed doesnt change, but as the load on the drivetrain (wind, hills, etc) changes, the cruise control WILL adjust throttle. And the mpg will move of course, since you are adding more throttle but no appreciable increase in speed

For me the torque of the diesel means nothing moves- just rock stable on the speed and throttle...but a smaller motor will need more 'adjusting'.

A