I'm disappointed about this. Polyphony, what? What??
You can change the final drive but not the individual ratios. It might seem picky of me it you're not totally into the game, but if you're trying for world-class lap times, minutely adjuting each ratio to best suit your chosen circuit for the maximum drive out of as many corners as possible is important. And makes you feel in charge.
Will this be added in a future update?