[Big Dave]

Reducing parasitic load has its virtues, but rather than haggling up an expensive water pump, underdriving makes more sense.

Of course I went at it the other way. With my numerically low gear ratios, my engine speed is down 35% at a given road speed so my water pump (and alternator, and A/C compressor, and power steering pump, etc) turn at a 35% lower RPM, drawing (pump and fan HP goes up with cube of RPM) 72% less parasitic HP loss. I’ve run this way for several years and had no problems with the auxiliary systems.

Reducing engine RPM works wonders for MPG.