[Diesel_Dave]

Interesting, however, I'm pretty skeptical of the 10% claim. I could see where the coolant could enable a gain, but I don't see where it would benefit in and of itself. Your coolant temperature is determined by your thermostat, not by the coolant itself. And the amount of heat that leaves the cylinders and goes to the coolant is determined by the cylinder wall temperature which, again, is not affected by the cooling medium. Also, max temperature is determined by the design of the block, head(s) as well, so you can't just switch to a different coolant and run 300 deg F coolant temps. If the coolant had a high heat capacity I could see where one could get away with a smaller water pump and save a little fuel, but that would require putting in a new pump. Also, a high heat capacity would mean that the engine would take longer to warm up, this loosing efficiency. A less viscous coolant might also help because the coolant pump wouldn't have to work so hard, however, I don't see anything getting much less viscous than water/glycol.

So, I guess I see how one might redesign the engine to work with a different coolant and see a benefit, but I don't see a benefit from just putting a different coolant in.