After upgrading your car with engines, suspension, brakes, etc., pause the game and go to "Performance Tuning". For better handling for the Lamborghini, Porsche, Corvette, and Viper, set the "Height" of you car to "-1". If you need more gripping power either take the "Height" to "-2" or set the "Aerodynamics" to "+1". If you go overboard with the performance tuning options (anything past "+3" or "-3"), the car will actually handle worse.