whatever the math is, I'm a lightweight guy on a bike with a higher headtube and bars than a road bike, 28s, a rear fender and rack, a helmet with a visor and a mirror--and I always get left behind on downhills by heavier riders combined with cleaner aero road bikes. Its frustrating because I am comfortable going fast so if there are turns and braking and whatnot I can easily keep ahead of people, but on straight downhills, it is very frustrating to be behind folks in a tuck and be dropped back.

So I figure its a double whammy, weight and aero.

there is an supported event my wife and I do usually every summer and you tend to see the same riders year to year. There is always a tall lady who outweighs me by quite a bit and this has happened with her and I-- she just laughed and said..."gravity my dear, gravity" as she knows her extra weight wins out. Each year when I see her I say hello in French "bonjour Madame gravity who descends faster than me" and she remembers it with a chuckle.