Once in a fast vehicle, there's an acceleration effect that adjusts the field of view to simulate head mouvement once accelerating or braking.
Unfortunately, this effect is bound to speed (if you go fast enough, like 200-300 km/h, your "head" starts going backwards, if you use the brakes, your "head" will progressively go forward).
It should be linked to acceleration (Gmeter) as it's more accurate. (Once fast enough, if you keep the same speed, your head shouldn't be pressed against the seat)
Tested on a Hatchback (sport). Haven't tested on other vehicles yet.