It’s probably been covered before but there are some solid numbers in this article:
"Compared with riding solo, the test rider gains a ~7W (3%) benefit from having her ride buddy directly behind her; a 76W (39%) benefit from drafting behind her ride buddy; and a 10W (5%) penalty when her ride buddy is riding alongside.
So in this experiment, I found a 3% energy demand benefit from having a trailing rider, and that’s right in line with (but at the top of) the range found by the other reported data referenced earlier.
This result of a 10W penalty when riding alongside another rider is more novel, although it doesn’t surprise me it may be news to some."
Thanks. Would be interested in whether anybody has tried to quantify this for larger road riding groups. Would have thought having a larger group of riders behind you would make for a bigger bow wave, particularly if those riders are less aero optimised (e.g. The guy behind you riding hoods not drops), and therefore maybe a greater benefit than the 2-3% found for 2-4 track riders in full aero gear. Personal experience seems to bear that out, but very hard to account for wind, gradient, road surface etc and actually quantify how much faster X watts at the front of a pack is compared to X watts solo.