Like drag, lift is proportional to area (but surface area instead of frontal area), the square of vehicle speed and Lift Coefficient (Cl), which is determined by the shape. At high speed, lift may be increased to such an extent that the car becomes very unstable. Lift is particularly serious at the rear, you can easily understand, since a low pressure area exists around the rear screen. If the rear lift is not adequately counter, rear wheels will become easy to slip, and that is very dangerous for a car travelling at something like 160 mph.