I'm doing a race next weekend where you aren't allowed to use tt bikes or aero bars. Historically for the final leg (a 70km flat ride) people just use 404's as the first 2 bike legs are hilly and they don't bother with tt helmets despite it usually being a solo ride. So currently my gear is an Enve 6.7 wheelset, a Giro Synthe helmet and a Giro Selector TT helmet which I wasn't planning on using but am now reconsidering.

So the question here is how much time/speed to I stand to gain if I use my aero helmet and a get disc from friends to put on bearing in mind I'm using a roadie set up?