There was an article recently that pointed out that your money is better spent on aerodynamic helmets than it is on skinnier tires/wheels.
But are they talking about differences in road tire widths (like 23s versus 25s) or are they talking about more significant differences, as between cruiser or mountain bike style fat tires and pretty much any road bike?

I have slightly fatter tires on the two road bikes I use for commuting and transportation than I do on the one that I just use for fun, and I'm not sure that any of the speed differences among those bikes can really be attributed to the width of the tires. But I can go much, much faster on my commute bike - a steel mixte weighed down with metal racks and baskets, and the most un-aerodynamic stem and handlebars you've ever seen -- than I can on my mountain bike, and I am sure it is because the mixte has skinny road tires. (They aren't even all that skinny for a road bike.) Switching from a hybrid to a road bike raised my average speed by about five miles an hour immediately, and I was wearing the same gloves and helmet.