MDS / Eco are different systems. The Eco light is just an indicator that the engine is running at high efficiency, it's just a feel-good dash light. It's not connected to the MDS, though. The MDS kicks on at low load with little to no acceleration, at speeds above 25 mph iirc. So you get to whatever speed you're moving at, the engine levels off around 1500 rpm, and MDS kicks on. MDS saves between 10% to 20% (tested) fuel when it's on, but that's only once you're at speed, of course. And it typically stops engaging at speeds around 65, depending on grade and wind resistance. I'm fairly sure that's mostly due to wind resistance being high enough that even at low rpms, the ECU is still sensing too much load. Going down a slight grade, MDS may kick on at higher speeds, even with a headwind. It seems to work mostly at speeds between 30 and 60. I find mine engaged a lot on country 2-lanes and not much at all on interstates. It does require oil pressure to run it - the lifters for the MDS cylinders are hydraulicly controlled. The Eco light is useless, best turned off. It goes on a lot when MDS isn't engaged, so it's not even a good telltale.