Ranking of some sort is a good idea, but i'd steer clear of using one figure to sum up a driver.
I think PB restrictions on servers might work. Ordering grids by PB worked wonders in the OWRL, some of the best and closest racing I've had. PB restrictions should obviously work both ways keeping slow drivers off fast servers and fast drivers off slow servers
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I believe LFS uses some kind of rating when you host a server, OK, QUICK, PRO, etc. Perhaps, that can be extended to WR+1, WR+2, etc only allowing racers on that have an appropriate PB. Since LFSW is already there why not use the stats there as a licensing system of sorts.
Most will argue that the only way you become fast is by racing other fast drivers. I agree with that but at times I'd like close competitive racing, not lapping alone on a server full of drivers out of my league
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