Looking for 'Real facts' answer to mileage question.

My truck is a manual transmission, And I have found that running it in a lower gear ( higher engine rpm ) it gets about the same fuel mileage, maybe 1 mpg worse ( 12 vs 11 ) but that water temp ( according to scangage2 ) runs somewhat cooler by letting it spin higher rpm. So my take from that is when running it in the higher gear, I was effectively "lugging" it. Mine is a gas engine that likes to rev anyway, so I choose the lower gear. I like seeing the water temp be a little lower.