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MrAngryBirds; Some good advices (and links) there, mate, but your summary is, imo, already decent if you're runing a friendly competition, just for fun, and people in the lobby can behave.
I would add to those, out of my head, 2 wheels out (and perhaps even clear straightening) would be a 0.
The main thing that I've learned from hosting a couple of comps - even amongst selected friends - is that people tend to get overcompetitive and no one enjoys losing, that's why, as they say, everything should be CRISTAL clear from the start. (and that's also one of the reasons why I stopped hosting them for a while).
Where to start, where's the speed matching zone/courtesy run, line/clips, and finish...(don't let people initiate like if it was a drag race, that's AWFULL and disrespectfull imo).
There's also the matter of "the other" things that tend to happen during these comps such as; crashes (whose fault was it ?), taps and pushes (whose fault was it ?), sudden lag jumps, etc etc..you know, those weird online issues that we GT player, are blessed with everyday. (BTW PD, when is this *enters track everyone has a lag jump* going to end ?).
Also consider some simple rules for people that fall out of the lobby during the comp (a time limit to get back on?). Also for people that simply decide to go and get pizza/popcorn/go take the dog out for a walk........on the middle of a comp...*HATE THAT!* (personally I sugest having a time limit for a player to get on track) and, if you allow that same player to do his/her run after getting back on lobby. You know, that sort of annoying things that ALWAYS happen when you don't want them to happen.
These last issues are the ones also - that from my experience - delllaaaaaaaaayyy a comp, make people unhappy, makes the whole thing go on forever and ever and are very important, as you probably have noticed, hosting comps online.
Good luck with the organization of it, you'll need it (and also a LOOOOT of time). Been there :/
EDIT: I also have one other tip; something I don't think I ever saw online (but im might be wrong?).
In case of doubt regarding a wheel (or wheels) off track, just ask a player on track (or go yourself) to go to that spot and (while spectating on that car) watch the
tyre marks on the floor. That should clear things out.