We're nearly constantly at loggerheads somehow with the issue of aerodynamics and the quantification of the benefits thereof. Inevitably, the benefit is expressed either as grams of drag at 50 kph, or as seconds saved per 40k at 50 kph. The 50 kph is so prevalent because that's where low speed wind tunnels give reliable results - below that, they get a little iffy. So even if the world's best cyclists can't ride at that speed for very long, equipment is tested at that speed. Such testing is valid but has constraints.
I've never actually ridden a 40k TT and don't really feel like making a habit of it, but I do spend a lot more time on a bike than I do in a wind tunnel, as do most of us. To tell someone that this piece of equipment saves some number of seconds in a 40k does a worse job of translating relevance to the rider than the wind tunnel does to the real world. "What does this mean in a crit, or a road race?" That's what most people want to know. The germ of the following idea started on the third stage at the Killington Road Race last spring. I'll spare you too many details but there were a lot of times that day when I felt like I was going much faster than I was going hard, so without actually having the cogent concept formed, I started trying to identify times where "the fast" exceeded "the WATTS." Having been on the hunt for this for a good long while without ever being able to articulate it beyond thinking about "moments of free speed," it hit me during last weekend's race. And now I am going to use a cheap literary technique that Mike and I criticize our favorite poorly written blog about how well the authors write about cycling for using all the time: the overly dramatic one sentence paragraph.
The way to think about advantages you get from aero gear or light gear or any kind of gear advantage is to think of it in terms of the number of matches it saves you.
When you hold huge speed through a close turn because your frame is tight, your wheels are fast and you can stay in the saddle and chill through the exit when other guys need to stand and hammer to nail the gap shut, you've saved a match. When you jump off the front just at that moment when the pack is ready to throttle back from "gone to plaid" speed to "will you guys please chill the f out" pace and hold free speed long enough to let you get an instagap and settle into the long torture, that's a big match you've saved. When your wheels spin up instantly when that skinny f--k who's been turning the screw all day goes on the climb and you latch right to him like tp on the heel of a shoe, that's a huge match saved. There aren't any matches getting saved at the end of the race, so that still needs to be measured in a different way - normally positioning and WATTS (capitalizing PRO serves the opposite purpose from the intended, but capitalizing WATTS is hip).
So, Mr Smart Guy, what does this mean? Maybe I will build a chart or a graph, or a chart with a graph. Until I build such a graph, words will have to do. For me, I seem to save a lot of matches with RFSW50s. I love those wheels because they corner like they should be illegal, they hold speed well, they're light enough so that my meager snap gets them going, you can roll a tight paceline in them in anything shy of a hurricane, and they just seem fast for me. 58s are pretty good in this regard, too. There's some overlap between 50s and 58s. If I didn't look forward to ripping the road tires off sometime in August and putting CX tires on, I'd probably go with 58s but 50s are a better go for cross. If you are constantly looking for more SNAP, then 38s will save a lot of matches. They go, and go, and go again, but then you're giving up something when you get to warp speed and want to stay there. 85s, well, a lot of the people for whom those are the right wheels HAVE done a 40k TT and might be looking forward to their next one and have no need to translate to anything other than seconds per 40k.
Next time I will tell you a funny joke, discuss why I think light weight is the underappreciated red-headed stepchild in the world of quantified gains, and tell you the phrase that made me absolutely crack up when I should have been throwing up during my last race.