What a question! It might be simpler to ask "how long is a rope?" as there simply is no one answer to this question.
In the simplest terms we can look at, aerodynamic performance of every wheel we tested suffered when the wider tire went on. There has been much speculation over this one recently, but the results of the tests we ran conclusively show that, in terms of measured aerodynamics, narrow tires are faster.
The question we were perhaps more intrigued to have answered was whether one rim or another tolerated wider tires better than others. Unexcitingly, the answer to that is also no; all rims suffered a similar drop off in speed when outfitted with 25mm versus 23mm tires.
Now, back to my "how long is a rope" question - how wide is a 23mm or 25mm tire? For that matter, how tall is either tire? As the chart below shows, that answer varies widely (I slay me) based on the rim to which it's mounted. The biggest determinant of inflated tire width and height (and thus inflated volume) is the interior width of the rim - the distance between the brake tracks. The relationship between interior width variance and tire inflated volume is steady in direction (wider interior rim reliably equals more inflated tire volume), but the magnitude of the change is not as perfectly predictable. For example, despite both rims having 18mm between the brake tracks, the tires we measured inflated bigger on Rails than on Pacenti SL23s. But a basic rough rule of thumb is that for every 2mm gain in width between the brake tracks, you will gain 1mm in inflated width. So if a tire of a stated size runs true to size on an Open Pro that is 14mm between the brake tracks, it will measure 2mm wider (which is equal to the most common size increment jump) on a rim with 18mm between the brake tracks. Which means that if you prefer a 23mm tire on a traditional-width rim, you can use a 21 on a Rail and get the same volume (more explanation of that to follow). And that, I promise, is the absolute last time I will mention an Open Pro in any discussion of aerodynamics!
The interesting part that follows on from this is that, when you measure two rims with the same tire, you aren't necessarily measuring the same tire on them. The 23mm Conti 4000s II that we used measured 24.3mm wide on the 404, but was a full 1.5mm wider on the Rail (and .4mm taller on the Rail, but to keep things simpler we'll focus on width). Similarly, the 25mm Conti 4000s II that measured 26.7mm wide on the 3.4 front measured 27.3mm wide on the Rail. Tires also set up relatively lower on the Enve rim compared to the width increase - the 23mm tire was .1mm taller on the 404 than it was on the Enve, despite the tire being .6mm wider on the Enve than the 404.
Given the negative relationship between width and speed, and given that tires measure bigger on our rims than on any others tested (which we knew they would - those who've followed the Rail story know that design parameter #1 was an 18mm interior width), we had to peel the onion back a little bit on that one. Interpolating the difference between 23mm and 25mm tires on the 404 creates a line that predicts where tires of widths between those two would fall. Create the same line with the Rail 52, and you see that for any given actual inflated tire width, the 52's "seconds saved" line is above the 404's. Of course we wouldn't be us if we didn't point out with equal emphasis that the 34's "seconds saved" line is below the 3.4's, so by using the same metric, a 3.4 is a little bit faster than a 34 for any given inflated tire width.
The current trend is absolutely for wider tires. Note that when we decided to test two tire sizes, we chose a 23 and a 25, not a 21 and a 23. Wider tires have been shown to have lower rolling resistance at equal pressure (don't worry, we're building a better mousetrap to measure that), and as many people have learned, offer advantages in both comfort and handling. Inflated volume also has serious ramifications for what tire pressure to use, which we will discuss in much more detail later, but the strange looks I've gotten for the past two years when I tell people what psi I use now make perfect sense.
There is a terrific amount of interrelated data that comes out of this, all of which will come out over the next several installments, but for now the myth (if there really was one) that wider tires are aerodynamically faster is busted.