Can Somebody Recommend a Racing Sim?

I'm in a bit of a pickle. I absolutely love racing games, but sadly I don't get to play that many good ones on the PC. I have TrackMania, and while fun, it isn't exactly what I am looking for.

My favorite racing sim is Forza 2. I just love the car customization, tuning, and especially being able to paint various designs onto my car which I would spend hours doing. The thing is though that I do not have Xbox Live and I really would rather not pay for it when I'd just be playing that one game on Live since no other Xbox game interests me that much.

So here's my question. Can anybody recommend a good racing game/sim with a bunch of customization options for the PC? I have tried to Need For Speed series in the past, but honestly it isn't my cup of tea and I really don't care for it. I have also looked at GRID, but you can't really customize much from what I have heard and there aren't that many cars.

Thanks!

The only 'simulation' racing title I can recall that's recent is GTR 2, released back in 2006 and got overall great reviews.

I've not played it personally, but there is a demo of it floating around on the internet, perhaps something to give a try in the meantime.

[EDIT] After some digging around, I can't find any mentions of being able to tune individual vehicle parts ala Forza/Gran Turismo in it though, so perhaps it won't be your cup of tea.

gtr2 - best racing sim in a really long time. race 07 is also excellent (by the same developer).