Yahoo Autos 
10 bolt-on auto performance upgrades that really work
MORE AT POPULAR MECHANICS
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.
High-Performance Shocks/Springs/Coil-Over Springs
Advantages: Tires stay in contact with the road on bumps and at higher speeds.
Trade-offs: Ride is stiffer, steering feels rougher.
The B12 Pro-Kit offered by spring-maker Eibach and shock-maker Bilstein combines springs and shocks that will slightly lower your car's ride height and are tuned to work together. A full setup for a new Scion FR-S would run about $1500.
Larger Anti-Roll Bars
Advantages: Car stays level with the road, optimizing tire contact patch.
Trade-offs: Bumps on one side are transferred to the other side, which makes for a rougher ride.
Front and rear anti-roll bars come in kits. One from Eibach for a Mustang GT runs less than $400.
