[elhigh]

Rule no. 1: the engine must run if the wheels are turning. Seriously. I don't know how many miles it takes to ruin an automatic, but the input shaft is integral to the lube getting around inside there while it's turning, so there's that.

Here's another thing: shifting into neutral in a rental Impala, I noticed my instant MPG would skyrocket, but usually not bury the gauge at its upper limit of 99. Coasting with my foot off the gas, but the tranny still in gear, the instant MPG would hit the upper limit and stay there until I got back on the gas. The reason:

The ECU was completely shutting off the fuel to the engine. The tranny was dragging the engine along at or above the cruise control setspeed, no more fuel was needed, and the engine was doing above idle speed, so no fuel needed there, either. The fuel injectors simply stopped injecting.

Your experience may vary, especially if you're driving a carbureted vehicle. This was my experience, and that was in a late-model large sedan that was, frankly, optimized to do its utmost with every gallon at cruising speed. For such a large car, 27.8mpg at 75mph is really very very good.