Why are aerodynamic benefits always expressed as watts savings at a certain speed when a much better metric would be % increase in speed?
Why do I ask that?
Well the number of watts that are saved depends on the amount of watts being produced in the first place which is obviously dependant on speed:
Lets say a rider was producing 360 watts at 45 kph (that’s a CDA of 0.3, assuming no other resistance). A change to the setup saves that rider 36 Watts at the same speed, or 10%. That’s a 10% reduction in CDA to 0.27. But who wants to go the same speed at lower watts? By keeping the power output the same the speed can be increased to 46.61 kph, a 3.57% increase in speed.
Now take another rider at 40 kph but with the same CDA. They are only putting out 252 watts. By making the same changes the CDA is reduced by 10% to 0.27 but the power reduction to hold the same speed is only 25 watts. Not such great marketing, even although the increase in speed is exactly the same at 3.57% (from 40 kph to 41.43 kph)
The reality is that a 36 watt saving sounds great to a rider with an FTP of 250 even although I’d never see those numbers in the real world.