In getting ready for Wildflower in 2005 I rode the CT course lots of time during the winter and spring before the event. I also did Wildflower back in 02 & 03. The CT course was much easier than the real course. Nasty Grade on the CT’er was not near as steep as the real climb.
I did Madison this past year and rode it on the CT and thought that the CT was quite a bit harder. After doing most of the loop once on the CT I was about ready to hit the panic button.
I felt pretty spent after two hours of the Florida course. I then did one loop of AZ the next day. Maybe because there’s no rest/coasting, it just seems a lot more difficult than riding outside. My power was about the same give or take a few percent as my PT, but the effort overall seemed much harder.
I did the IMAZ course on the CT a bunch of times before last year’s race - and found my real bike split was almost dead-on with my CT split. I think I was within 1 minute of my average training ride time.
For IMC - I was 15 minutes faster in reality than I was on the CT.
For CDA - it was it was close to being a similar split, but off a little more than IMAZ.
My guess is that it has to do with how many hills are on the course - both uphill and downhill. On the CT you end up pushing on the downhill and you don’t see the watts backing off as much as you do in the real world on a downhill.