This is now the $64,000 question for all ITT and triathlon competitors - which wheelset is going to give you the best performance/time savings....Here's what I have found.

I spoke with folks from Hed and Zipp - definitely two of the top wheelmakers in the business and got some good opinions backed up with great data.

In a nutshell: given low wind conditions, if your speed is less than 25 MPH, tri spoke wheels can give you up to a 15 second advantage over 40K. They have better aerodynamics at lower speeds, lower drag coeficients, and create less turbulence - most of the time. At speeds above 40K, the deep dish becomes the better choice - the main reason is that the spokes have less contribution to drag and turbulence at higher speeds.

When the wind is high though - then all things change. A shift of 5 deg can make both wheels a disaster. If your course is full of crosswinds - opt for a shallower rim for handling and speed. If it;s headwinds - straight on or you can point straight on, then the first rules apply - based on speed.

Dimpling - This gets really complicated - Zipp stands by it, Hed says it's not really worth it. My thoughts after reading this article on aerospace. org  about golf balls that the extra 1-3% dimpling 'might' give you, and/or the extra handling ability you 'might' get in the wind is well worth it. 1 second wins a time trial.

Lastly - accessories - based on an article by the MIT cycling team (nerds going fast!) an aero helmet gives you almost as much performance advantage as an aero front wheel, lose the gloves and gain 3 seconds on 40K, shave the arms get 1 second, wear booties, it's worth 2-4 seconds. Trim you cables, shine your bike, clean your chain, remove your speed sensor (keep cadence), look for things on the bike and yourself that will make the wind harder to pass over you smoothly. Work on your position - video and shoot yourself. flat back, aero hands and arms, open diaphragm.

Bottom line - in a time trial - EVERY SECOND COUNTS!!!!