Don't think you can find it anywhere. Sounds like something you could make up yourself quite easily though.. I would say:
(sum of all car parts performance level / 7) + (sum of all car parts wear level / 7)
--------------------------------------------------------------------------------------------------------- = car performance level
2
Disclaimer: there's probably a ratio between parts performance and wear in relation to the car performance though (instead of 1:1 in the equation above). Ie. I can imagine that the wear has a smaller impact on the overall performance than a badly designed front wing. You would have to reverse engineer this, but that's not easy to do, so I haven't yet taken this into account.
Just realized I left out engine wear from the equation too by the way;
(sum of all car parts performance level / 7) + (sum of all car parts (including engine) wear level / 8)
----------------------------------------------------------------------------------------------------------------------------------- = car performance level
2
But yeah, I think maths can be fun.. especially when it becomes concrete, like with iGP.
I guess your performance is always relative. You have to compare to the other drivers in your league. If you use Rene his method and you find that you suck you can still be the best of your league if the rest sucks more.
So your average performance is your result after every race devided by the number of races. Or you use the standings button and you'll see the where you stand.