as mentioned knobby tires waste power... so simply going to a smooth center tire will help things out...
going to a smaller tire does have some other issues... the smaller the tire the higher pressure is needed for ideal drop
http://www.dorkypantsr.us/bike-tire-...alculator.html
at your weight + your bike out back you are looking at about 115psi if you went to a 32c tire but 145psi with a 28c... if you wanted a 23c you're looking at over 200psi...
I'm slightly heavier then you and am running
Panaracer Pasela 32c tires they run nicely at about 110 psi its prob worth spending a bit more for the TourGuard version as mentioned above for the extra flat protection...
that being said going with a 38c tire with a smooth tread in center will help out a lot on the road... I rand some kenda kross tires in both 42 and 38c happily on a bike that mostly saw the road but occasional gravel road/greenway