Originally Posted by datlas

But on today's ride, I noticed that my fellow riders were really out-descending me. By a few MPH.

It appears that, if you assume negligble rolling resistance and that coefficient of drag are the same for each rider (which is a leap, bigger riders are likely to have more frontal area and thus more drag), that terminal velocity while going downhill is directly proportional to the square root of the mass of rider + bike.

Therefore, assuming that my fellow riders + bike weight are 20% more than me (which I think is accurate since my weight + bike is around 170 and theirs is around 205, which is 20% more than mine), their velocity going down a hill is roughly 10% faster than mine (square root of 1.2 is 1.095).