Depends on your experience, engine (strength and Vo2 Max), the course, the conditions as well as the specific distance of the TT. The distance of the TT event is not always 40k...the one that I raced in during spring was 10.5 miles in a 7-race series. The course was not a cake walk as it had hills, a 180 turn around and wind. I averaged 23-24 mph over 7-races and was a mid-pack guy in the 30+ cat4 racers at the age of 47 on the best equipment $$ can buy. I average pretty much the same speeds in a 1/2 IM (23-24) as I'm not a sprint guy at all but more an endurance athlete. Although, I do pretty well on the bike in 1/2 IM's and IM's the pro's are are 4-5 MPH faster. The top guys in the TT series I raced in are 27 - 28 mph avg speed, but these guys are real cyclists (seasoned Cat 2 and 3 racers) not swimmers or runners. They are on their bikes every day putting in a ton of miles and they have been doing it for years as part of a competitive team.