This question comes up periodically. I’ve always known the answer: it’s substantially quicker to ride a TT bike in almost all races, with the only exception being hill climbs.
Today i opted for my road bike with no aero equipment at all and pitched up at a rank bongo-fest near Marshfield. I did this for training purposes. Interestingly, it took the pressure off to ‘do a ride’. I found it much tougher on the road bike and lost oodles of time on any flat or downhill section, in fact, on anything other than a particularly steep wall. There weren’t any steep walls.
My maximum speed was 39mph, average was a shade under 23mph for the 22 mile circuit with 1500ft of climbing. Average heartrate was 168bpm, it says max was 210bpm but this is a damned lie.
I was around 4 or 5 minutes slower than i might have expected if i’d come in fancy dress. It’s a bit like turning up to drag race on a tractor. Perfect for tilling the fields, not much good at flat line speed. Nevertheless, either side of and including the race, i managed 61.5 miles at 20mph average speed. It’s all a desperate attempt at training and endurance ahead of a longer race at the end of the month.