Ah, Grasshopper, you have stumbled upon one of the basic bones of contention amongst cyclists -- the "Which Tire Is Fastest" conundrum!!!  Be VERY, very careful when you open this can of worms...  
 
In my couple of years of Randonneuring (a French word, meaning "Insane Miles"), basically, I'd have to say that changing road surface conditions FAR outweigh effects of particular tire comparisons.  Going from smooth pavement to fresh "chipseal" and back will result in a speed change of several (2-4) mph alone.  
 
 I'm not sure myself where to find "official" rolling resistance figures, but the "conventional wisdom" is thus:
 
*  Wider, somewhat lower pressure tires roll better over rough surfaces than high pressure skinny ones; For fastest riding, most people prefer skinny, high pressure tires, but they pay for it on rough surfaces through a rougher ride.  
*  Sidewall flex is a contributor -- a stiff sidewall creates more rolling resistance;
*  Tread also creates rolling resistance, and tread isn't really needed on a bike for road riding, since you cannot get a bike up to hydroplaning speeds -- especially since the contact patch to the road is oblong.  So go with "slicks"...  (What's a "City" tire?)
*  Rotational inertia also comes into play, especially in racing where acceleration counts.  
*  Spokes also contribute to aerodynamic resistance, which is why people utilize disk wheel covers.
 
Ultimately, for a "Utility" vehicle, include a Kevlar belt or some sort of "puncture proofing" in your tires, whatever you pick.  Because flats happen, usually at the most inconvenient time -- and getting a wheel off of a loaded trike creates its own issues.  
 
Ah, mechanical engineering...  those were the days...  
 

Quote from FlyingLaZBoy on Jun 9^th, 2009, 7:36pm:

 
  yes the topic has been driving me crazy. I really like the Kevlar belt suggestion. We will keep that in mind.  
 
And by city tire I mean a tire that has some tread so it's somewhat more puncture resistant than many road tires, but it is not as knobby either as most mountain bike tires. I would insert a picture here which would be a lot easier, but I haven't had time to set a photobucket accnt up yet.
 
Thank you for spreading the wisdom  
 
Lesley

Quote from UofHLesley on Jun 12^th, 2009, 1:17pm:

There are road slicks that have much greater puncture resistance than treaded tires which have thin spots by their nature. At least with a road slick you have uniform puncture resistance or vulnerability. Unless you are planning extensive off-pavement excursions stick with the slicks. Getting a grip on uneven surfaces is the only engineering reason for treaded bike tires, and there are so many arguments against treaded tires. If you're building for the ASME competition their rules tend to favor trikes, and the physics of trikes trend to favor small wheels with fat tires unless special hubs or wheels are used.