Good question, depends on where the mpg improvements comes from. Is it less effort for the water pump, power not needed for the electric or mechanical fan, is it allowing more aero mods or is a poorly cooled engine able to maintain and run advance vs retarded timing?
I use water wetter in my insight and I think it helps to run the grill block I do and maintain. For the hell of it I ran with a full block last summer and I got better mpg having the cooling fan run vs having enough air flow for it to maintain on its own.
