Measuring your average speed over 5 miles barely tells any kind of picture, other than to prove, surprise surprise, you were quicker on a road bike than on your MTB…
5 flattish miles, on a nice day, no wind, head down, feeling fresh… Could get 25-26mph avg for sure.
Back in reality where my rides are longer, conditions much more variable, and hills exist, avg ride speeds vary from 12mph (60 miles in the Peaks in December in sleet and snow!) to about 19mph (Our Thursday night training runs peak around this pace in the middle of summer on around 30 mile loops). But I’m merely very average on a road bike, have no pretensions of grandeur, and am not about to cut out the pies/chips/cider to lose 2 stone to make me a competitive weight as I just enjoy riding the things!