Haha Jack, just as I was about to say something smart you steal my lightning.
True the lower rpm would suggest better mileage but the other factor in fuel economy is engine load. A higher engine load can hurt your economy more than a slightly higher rpm. I actually tested it about a month ago to show a friend of mine who was thinking the same thing except he was completely convinced that putting 35s on a chevy colorado with an I4 and highway gears would get him killer mileage because of the final drive ratio.
Here is the thread that has the graphs from compiling multiple recordings using the gryphon's data log feature.
http://forum.gopowerhungry.com/conve...t-mileage.html
It is easier to turn a gear twice as many times with smaller tires than to turn a heavy set of tires for all of (and more) factors that Jack has just mentioned.