These days, cars are so optimized that it's usually not possible to improve over the factory designs. We said "usually"—we've found 10 relatively simple bolt-on components that give you true performance increases without permanent modifications to your car.
[Related: Six ways to ruin your ride with terrible add-ons]
Wider Wheels and Tires
Advantages: Improved road-holding, turn-in precision, and braking.
Trade-offs: The extra weight can overwhelm shock damping and springs, so loose-surface tracking stability is reduced.
The easiest way to improve handling is to "plus-size" your wheels and tires (a term coined by The Tire Rack). It means proportionally reducing sidewall height, increasing wheel diameter, and increasing width to ensure your wider tires keep the same diameter. Prices start at about $1300 for plus-sizing a Ford Focus.