So I've done quite a few 1:1 endurance races in AMS2, from IMSA style 100min "endurance sprints" with a single driver change, to full 24hr races with a driver change every few hours. No matter what I'm driving, the AI co driver will almost always be a second or so slower than me which can be pretty frustrating. So your co drivers pace is tied to the in game difficulty. So to speed them up, what you want to do is go into your custom AI file, and move the race/quali skills for every driver back by about 5-10 points, then raise the in game difficulty a few points. Since the in game difficulty is raised, everyone should be doing similar lap times even though there's skills are reduced in the AI files. Your co driver should now be much more competitive. Hope this helps. If you've never done real endurance racing in AMS2, I can't recommend it enough. No other sim can get close to this level of immersion.
