The one convention we have left to cover following last week's wind tunnel test is the "how many seconds will I save or spend in the mythical 40k TT by making any of the above choices?" Since the results so clearly deserve a different take on it than what's been presented in the past, we're going to express it in terms of distance rather than time.
Using a 303 instead of a Kinlin XR31T/FSW3 or an AForce Al33/RFSW3 will put you 40mm (we originally said .4mm - Mike carried the 2 wrong somewhere earlier, and an eagle-eyed commenter caught it) ahead after 40k. The construct here is that the 303 is ridden at a power that makes the rider go 25 mph, and the others ride at that same power. The FLO30 and HED Belgium+ are a couple of bike lengths behind. That's it, and that's the extent of our summary report there.
Maybe we just magically picked the 5 wheels where this would occur? Maybe our distribution (which again, is something of a distribution of distributions) is a bit off? We can't help but concluding that if you choose any good, modern wheel of some moderate depth and width, you're putting yourself at no aerodynamic disadvantage with the (possible) exception of in high level TT competition.
There are some other differentiators, though. One is rolling resistance. Your rims don't make any real difference there, but your tubes might. And latex tubes have been shown (note that I didn't use the word "proven" since some of you are already screaming "but that's not a real world test!!!!") to have lower rolling resistance than butyl tubes, and the delta is bigger than the aero gap seen in our test. And rolling resistance doesn't decrease when you draft. If you use butyl tubes, there's a range of rolling resistance there, too (same link as above).
Tires make a difference too. Much bigger than wheel aerodynamics. Just yesterday, I read some guy on a forum that he could clearly feel the difference when he switched to his carbon clinchers versus his other wheels that have Gatorskins on them. He didn't say what tires were on his carbon clinchers, but it's not at all unlikely that there was a 20w difference in the tires he's using - so OF COURSE he can feel it. And this is likely to be the "noise" in the usual anecdotal comments like this. Our guess is that people had always put the garden hose tires on the alloy training wheels, with fast tires on the carbon race wheels. Now that people are sharing great info on rolling resistance and people are paying more attention to it, it's likely that the tires were making the difference, yet people blamed it on the wheels. Isolate your variables.
So, within wheels, what does make a difference?
Looks make a difference. I mean let's face it, carbon looks pretty freaking cool. If carbon happened to be really ugly, would people use it? If you dig deeper or shallower wheels, that's going to make a difference to you. We've plainly stated before and will plainly state again right now that getting a Special Edition matte finish on our XR31Ts exponentially increased our enthusiasm for what was otherwise already an easy rim to love. And then there's the whole "ceramic coated Al33s sold out in 4 days" thing. So go with it, and don't feel guilty about it.
Price makes a difference. You could pay for an entire season of race or gran fondo/century entry fees (with enough left over to buy fresh tires all year long) with the price gap between FSW3s and 303s. Having money left over to not think twice about saying yes to an event you want to do makes a difference. You can put a Powertap into a set of RFSW3s and still save most of a grand from a lot of carbons. Training with power helps you make a difference.
Handling makes a difference. Not getting blown around in crosswinds makes a big difference. Tire set up and cornering makes a big difference (never forget that the impetus behind the Rail series was width more than anything else). Having a front wheel that holds a line makes a difference.
Weight makes a difference. I'll get skewered for saying that, but "light and stiff" are the two most popular answers when we ask people what they're looking for in a set of wheels. They often exist on competing curves, so getting the right mix of both is a compromise, but we're able to do it with PLENTY of builds.
Hubs make a big difference. We've said it for years and years - buy hubs first. You won't roll any faster out of the box with fancy hubs, but good hubs will see you through several sets of rims - rims are a wear item, hubs don't have to be.
And finally, build quality makes a huge difference. When you install your wheels they should be silent, round, and true, with nice even tension on the spokes. And they should stay that way for a good long time. If the builder has spent some effort helping you discover what mix of components will work best for your use, you should be able to ride them for a long long time without doing much more than keeping them clean (WITHOUT using a pressure washer!!).
Good thing I wore my kevlar underoos today because I have a feeling we'll take some heat for such heresy.