lex,
I know what you mean, I use keyboard too. The reason is that because its digital, steering is either full lock or straight ahead. Therefore the game compensates for this and helps you stay on the racing line slightly, and smoothing out your steering.
It works quite well, and is the best solution to the problem. However, I find gp3 2000 is the best for keyboard users, better than gp4 in that respect.
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She says brief things, her love’s a pony
My love’s subliminal