Peter, I don't believe that the reduction of the air density created the increase in fuel economy. The engine would reduce the available fuel, in similar fashion to adjusting the ALDA clockwise, to compensate for the reduced air density. The ALDA is hardly a precision device, so the reduction in fuel may be more than......or less than required for the reduction in air density. In any case, the ratio of fuel to air is different at the increased altitude. It might be richer or leaner than what you have at sea level.

Therefore, to duplicate such a situation would require an adjustment of the ALDA.......either richer or leaner.........the direction is unknown. You'll need to experiment and determine which direction the engine prefers. I'll take a guess and say that it should be adjusted richer (counterclockwise). Give it a try and turn the screw 1 turn counterclockwise. Then monitor the fuel economy carefully over several tanks and report back with results.

Make sure you have some good baseline data before turning that screw.

Just as a reference, the SD just got 31.1 mpg in mixed driving (10/90). The ALDA is richer, the diff is a 2.88, the a/c is off, the tires are at 34 psi, and the speed is kept between 65 and 68 on the highway. I also use 8 oz. of Power Service to approx. 13 gallons of fuel. I don't expect the SD to duplicate this result, but, I'm fairly confident of repeating 30 mpg. I got 29.5 on the prior tank.