The overwhelming majority of Mustang's were designed only to quickly accelerate in a straight line. The rationale was that handling was an important feature for sports cars rather than muscle cars. As a result, Mustang's tended to perform well until the vehicle had to be turned, in which case the familiar screeching sound of the tires lifting off of the ground was almost guaranteed to be produced. Although modern Mustang's handle significantly better then the early models, almost all Mustangs can use help with handling. Thankfully, the aftermarket provides a virtually endless supply of handling upgrades for Ford's Mustang line.
Replace the stock tires with tires featuring wider tread. Tires are one of the Mustang's most important handling components. Narrow stock tires have difficulty grasping the road surface, particularly when cornering at high speeds. This is due to the tire's small surface area. Increasing the tire's width with increase the surface area and allow the Mustang to turn at a greater speed without the familiar screeching sound produced by narrow tires.
Install larger front and rear sway bars. Sway bars are designed to minimize body flex when the Mustang is turned, particularly at high speeds. The smaller the diameter of the sway bar, the greater the body flex. Too much body flex reduces handling because the tires will have a tendency to lift off the ground due to body sway. Ford manufactured the majority Mustangs with relatively small sway bars, so increasing the size of the sway bars will significantly increase the Mustang's handling.
Lower the rear of the Mustang to reduce the amount of air which travels under the vehicle, thereby reducing drag and the Mustang's center of gravity. Mustangs can be lowered by simply replacing the stock leaf springs with leaf springs which feature repositioned spring eyes. A reverse spring eye features an eye positioned underneath the leaf springs as opposed to on top of the springs. Reverse eye springs typically lower a Mustang by 1 ½". Mid eye springs are also available, and generally lower the vehicle 1" below the stock height.
Lower the front end of the Mustang by installing shorter and firmer coil springs. Aftermarket coil springs typically lower the front of the Mustang by 1", and the combination of a lower suspension and firmer spring will dramatically improve the handling.
Replace the stock shocks with quality aftermarket shocks. Shocks are designed to minimize the effects of an imperfect road surface from the interior of the vehicle. Stock shocks are notorious for being "soft," meaning that when the Mustang travels over bumps in the road surface, the car has a tenancy to bounce. This bouncing effect may provide a more comfortable ride, but handling is severely impaired because the tires will not be able to grip the road as well at the top of the bounce. Installing shocks which reduce this bouncing effect, commonly referred to as "stiffer" shocks, will maximize the Mustang's ability to grip the road.
