Everyone above is right, but are missing a key element
It is about exit speed as it is on every game and indeed real life racing. But in this game, like other sims, it's also about tire temperatures and being in the optimum temperature zone. If you are running around on 70C tires and I'm running on 95C tires, and we are running the exact same lines and inputs, I'll be significantly faster than you. You must learn to tune for and manage tire temperatures first and foremost, as all tuning depends on it. In my garage I have some videos posted. The Clio at Donington video comes with some pop-up instructions for maintaining speed and shows you the kind of racing lines you need to take to be quick at that track.
The Clio was difficult to tune as the front tires kept overheating so I'd usually only get one hot lap in. I could fix it with higher front pressures but for a hot lap, I only need one lap. FR's and MR's are more consistent with tire temperatures. Camber, brake duct openings, tire pressure and even sway bar settings can affect tire temperatures, along with track conditions and how you drive. If you are swining the back end out quite often you will heat up the rears. Plowing the front end or over rotating the wheel will heat up the fronts.
Another trait of this game is that breaking traction at the rear costs you time. It's fun, it looks great on replays to carry huge slip angles, but it's slow. There is an optimum slip angle for all car/tire combinations and you can feel it when it's right, but when you go beyond that, you lose forward momentum. If it's happening consistently on the track, you have too much oversteer and have to adapt your driving style and/or the tune to avoid it as much as possible.
Last edited: Jun 18, 2015