There is no 6 cylinder mode in the MDS system. It is always either 4 or 8.
As for it's usefulness. I notice a reliable upward bump on the instant economy graph when it engages. I got back from a trip to CO on Friday, and got the following for the trip:
Tank 1 - Williston ND to Gillette, WY. 19.3 mpg with headwinds and a hilly drive
Tank 2 - Gillette WY to Aurora CO. 21.7 mpg with no head or tailwinds and hilly drive
Tank 3 - Aurora, CO to Gillette Wy. 17.3 mpg with 35 mph plus headwinds, blowing snow, icy and snowpacked roads requiring the use of 4x4, and pulling an empty tow dolly, and hilly drive.
Tank 4 - Gillette, WY to Williston ND. 20 mpg with a mix of head and tailwinds, towing the empty tow dolly, and hilly drive.
My truck has just crossed the 5k mile mark on the return side of this trip. Speed on the way down was 60 mph, and the drive home was capped at 55 mph when weather permitted to keep the dolly from swaying.
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