This is a big stickler with my xD, it really irks me - I get between 24 and 26 mpg around town and driving back roads to work, and the best I've ever gotten on the highway is 29 - but perhaps somewhat because I pretty much have to drive between 75 and 85 mph on the highways around here - If I tried to drive anywhere close to 60mph I would be road kill for sure...
45 mpg is just totally incredulous to me comparing with my results... Grrr!