The different drive modes affect throttle response, shift strategy, torque converter lockup, and AWD engagement. All of those affect gas mileage. In the final analysis though, gas mileage is purely a function of brake specific fuel consumption (BSFC). That number varies based on load, rpm, and cam phasing. So, getting to proper speed faster is not a strategy that one should use. It obviously takes more hp to achieve a specific speed in a shorter time (acceleration) and this would require that the BSFC numbers at those hp levels are so much greater that they make up for the extra HP used. On a modern cam phased engine, that is not likely. Also, the ECO mode upshifts earlier, most likely locks up the torque converter earlier than NORMAL mode and limits engagement of the AWD case. That's why the ECO mode is best for gas mileage.