According to some studies, aero wheels only reduce wind drag by 10 to 20 watts compared to a 28-spoke v-shaped rim, which isn't much. This study says that $200 Mavic Aksiums are only 12 watts less aero than $3000 Zipp 808s at 50kph (less at lower speeds, depending on wind direction), and only 9 watts less efficient that Cosmic Carbones: http://www.rouesartisanales.com/article-4934445.html And this calculator suggests that an increase of 12 watts output around 45kph only increases speed by 0.5 kph: http://www.kreuzotter.de/english/espeed.htm
I've spoken to lots of club racers about their Cosmic Carbones and Zipp 404s, and the conservative guys say that the speed difference seems like 1 or 2kph at good speed (~45 to 55kph), and the optimistic guys say about 3kph at speed. Those claims obviously contradict the Kreuzotter calculator. If I was convinced that I'd be getting a 3kph advantage at, or around 45+kph, I'd definitely buy some fancy wheels, but for 1kph, I won't 🙂
These numbers from Tour Magazine suggest that 2 tri-spoke wheels reduced drag by 15 watts, and I'd say tri-spokes are likely to be more aero than a 20-spoke 60mm deep Zipp wheel, for example.
Required output to maintain 45 kph on a:
Standard road bike, hands on hoods = 465 Watts
Same bike, hands down on the drops = 406 watts
Same bike with aero bars = 369 Watts
Same bike, triathlon position (5.5 cm lower bar, saddle forwards)= 360 Watts
Same bike, as above, with 2 tri spoke wheels = 345 Watts
Cervelo TT bike with 2 tri spoke wheels = 328 Watts
Cervelo TT bike with tri spoke front & disk rear = 320 Watts
Cervelo TT bike, tri spoke front & disk rear + aero helmet = 317 Watts
Cervelo TT bike, same as above + skin suit = 307 Watts
Cervelo TT bike, same as above, with saddle pushed back 3cm = 293 Watts
