We have a 2005 xB, maybe I can give you the information you're looking for. Our year model came with (probably an optional) rear "wing" or "spoiler". It directs the wind from the top of the vehicle straight down across the back window. We hardly get any dirt build up on our back window. Ours looks like this...and is OEM from Toyota/Scion
I don't know if this "wing" is an option for your year model or not, but there are aftermarket "wings" very similar to the one we have. Hope that helps.
On the MPG discussion, our 05 xB with an auto transmission regularly gets 34-35 MPG. It is completely stock with no mods at all. My wife is the primary driver and most of her driving is down a 45 mph highway with very few redlights. She is a very good driver and has always gotten great mileage in all of our vehiclels. Though the vehicle may not look very aero, it definately performs at the pump and the room inside is phenominal. Don't knock the xB til ya try it, it's a great little vehicle.
