My wife has a 2007 TCH, the highest mileage on a tank has been 46.2 using cc, ac, & eco mode at interstate speeds. She has mud guards it. Now what the mileage might have been without them, who knows/who cares.

 

You'll see a definite drop in mileage if you lower the tire pressure enough so the mudguards are dragging on the ground.... just kidding!

I've had mudflaps installed when I took delivery on my last five cars. A few trips on gravel roads taught me that this is cheap insurance to avoid pockmarked rocker panels and lower rear quarter panels.