coachboyd

Cervelo actually did this testing (with bikes though). Because a slower rider is out on the course longer, they actually get more of a benefit from aero equipment. There are three forces to overcome when you are riding, friction, gravity, and aero. Friction is a very small percentage but is consistent no matter what speed you are going. Gravity forces only are affected by going up hill or down hill, so when you are on flat ground there are no gravity forces. The rest is aero forces, and even at slow speeds it's the majority of what you are overcoming when you pedal.

Now, going more aerodynamic will make a bigger difference to have an overall faster speed. And to increase your speed you are going through exponentially higher aero forces (which is why it's harder to go from 28mph to 30mph compared to 22mph to 24mph). But the rider who is going 22mph will be on course for a lot longer than the rider going 30, and if each rider changes exactly the same things the slower rider will have a bigger time savings.

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I just plugged in some calculations.
A rider who average 300 watts for a dead calm, dead flat time trial at 100m above sea level and a drag coefficient of .330 (rolling resistance of .005) will average 40.20kph and have a time of 29:51. If he keeps all else the same but lowers his drag coefficient to .315 (the difference of a good set of wheels or not) he will average 40.79Kph for a time of 29:25. The faster drag coefficient will save him 26 seconds over 20K

Now, take another rider with the exact same drag and conditions. If he averages 200 watts for the 20K effort he will average 34.5Kph and will have a time of 34:46 for the 20K time trial. Again, if you improve the drag coefficient from .330 to .315 and leave all other variables the same the rider will improve to a speed of 35.0Kph and the time will be 34:17. This rider will have saved 29 seconds over 20K.