For decades, the conventional wisdom has been that smaller / skinnier tires = a faster ride. The though process was that smaller / skinnier mean less drag / wind resistance and a smaller contact patch on the road–so, less friction. And as a result, 23mm tires have been the default setting on road racing bikes–and road bikes in general–for decades.
Recently the trend in professional racing has been away from 23mm and towards 25mm tires.
http://www.competitivecyclist.com/learn/25-vs-23
Out of about 70 pre-owned road bikes of various vintages and price points in my inventory at any given time, only about two or three of those bikes came from the factory with (or currently have) 25mm or 28mm tires on them. The rest have 23mm. (Even the non-race / endurance / touring bikes–like the Specialized Roubaixs and Trek Pilots–which aren’t really intended to go fast often come with 23mm.) This goes to show you how pervasive the 23mm mind set has been.
On my own personal road bikes I now roll strictly 25mm tires… and I’ve even started putting 28mm tires on my road bikes that are used more for touring or entry level training or social riding… (though not all frames and / or brake calipers will accept 28mm tires).
Continental Gatorskins–arguably one of the best road tires available–are now available in 700x 23, 25, 28 and even 32mm widths. If you are not a speed demon or a weight weenie (obviously wider tires will weigh incrementally more than narrower ones), you’ll find that a 28mm tire–at full pressure–rolls nearly as fast–and is a LOT more comfortable–than a 23mm tire.
And–particularly when training–speed is not important as WATTS –or distance. So, the next time you need a set of tires… you might want to go with a 25mm or 28mm version instead of the 23mm that are probably on your wheels currently. You’ll roll just as fast–if not faster… and you’ll feel less of the road transferring up into your ass and arms through the saddle and handlebar. And less vibration transferring into your body, you should have more endurance and less fatigue. (If you’ve ever used a jackhammer, you’ll understand why.)
(The width of your rims can certainly be a factor in this decision as well… putting a 25mm tire on a 19mm rim create some additional drag (wind resistance) at higher speeds that could offset the lower rolling resistance of the wider tire… so, in terms of net speed gains, those two might cancel each other out… but the ride quality should still be noticeable better (smoother) with a wider tire.
(On a similar note… Sheldon Brown has some important insights into bicycle tire rotation in this article: http://sheldonbrown.com/tire-rotation.html)
Also on a similar topic:
https://janheine.wordpress.com/2013/05/23/tire-width-how-much-difference-do-a-few-millimeters-make/
And another from Slow Twitch about rim and tire widths:
http://www.slowtwitch.com/Products/Things_that_Roll/Tires/Choosing_your_tire_width_4026.html
And yet another good article on the subject from Cycling Weekly:
http://www.cyclingweekly.co.uk/news/latest-news/are-wider-tyres-really-faster-160403
And some new info from VeloNews (Oct 2016):
And a recent scientific comparison of 23, 25, and 28mm Continental GP 4000s II tires yields some interesting data:
http://www.bicyclerollingresistance.com/specials/conti-gp4000s-ii-23-25-28