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I wouldn't go below 250cc for freeway work. Your top speeds are a bit optimistic - those might just eek out those speeds in a full tuck on level ground with no wind. You'd be lucky to be able to maintain 55mph on a 150-200 in the real world though.
I often commute on my wife's DR250SE enduro bike and I definitely wouldn't go any slower. It'll maintain 60mph all day reasonably comfortably and still has ok passing power and will run up to 75mph without straining too badly (as long as it isn't pointed up a hill or into a stiff wind), but you wouldn't want to do 75 for more than a few miles at a time. it is very smooth and efficient in the 35-50mph range.
You might also consider the ninja 250 as its faster and still quite efficient, the buell blast (quite efficent, but not the most reliable thing ever made) and all the 250-400cc range enduros. Even some 650cc enduros get quite good mpgs. A stock honda XR650L or BMW F650GS (old single cylinder models, not the new 800cc twins) will usually do 50-60mpg with normal riding.
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