Sorry about my late entry - I just found this thread.
Lonesome Trail, welcome aboard from a fellow Mustang owner and Coyote owner and Arizona resident.
In my experience, the old hot-rodding trick to increase
available power to the wheels by under-driving the accessories
does result in better fuel economy. It results in more power available to the wheels throughout the entire operating RPM range.
Last May, I installed an ASP under-drive crankshaft pulley on the 4.0 liter Cologne V-6 in my '07 Mustang. It under-drives the accessories (particularly, the water pump and power steering pump) by 25%. It gave me close to 1 MPG, according to my fuel log.
Most people don't believe that I can get over 32 MPG with a 4 liter Mustang.
