[38Chevy454]

I have the ISX (with 450 hp tune) and the Eaton Ultrashift 10 speed. Shifting is pretty quick, but not automatic quick. With no clutch pedal in the cab it really is easy to drive and for the most part the computer picks the right gear and shift points, both up and down. The only time it can get wrong is coasting at lower speeds it will sometimes pick 1 gear lower than optimum. Not really a big deal, it just upshifts real quick. Remember that the bigger the engine (ie ISX 15 liter) it has less rpm range and will work better with additional gears to keep in optimum range. No matter what you will not win any races taking off from dead stop.

I have driven all over CO, major interstates to 2-lane highways in the mountains. Over 11,000 ft passes and towing an open car trailer or four down. I find the power is sufficient, sure more can be better. I especially like the three-stage engine brake for controlling descent after cresting the mountain.

I can't say if mpg is different manual vs slush box. I am geared so 1500 rpm is 70 mph and that becomes a nice cruising speed for me 70-72 is typical. I get about 7 mpg and not sure physics would allow that to be much different with either trans. You have a certain aero drag and weight, and that takes a given amount of fuel.