vaccum modified cruise control linkage
 

if that doesn't describe it enough
What picture comes to mind?

Hum you want an autopilot for your dyson? Just get a roomba and be done with it :D

ORRRR

Maybe you want a cruise control that keeps constant vacuum instead of constant speed?

My thought last night was wrong. I wanted to use vaccum to slow down cruise control when low, and speed up when high. Rather an automatic hypermiling device
This would have to be done with electronic changes to speed control either modify the speed sensor signal, or modify the speed selector switching automatcally
maybe just set the cruise control to the gas mileage sensor device instead of speedometer
give throttle position priority over speed
Again that wouldn't work directly,
But the idea is to set mpg instead of mph