does a body kit and other accessories add speed to a car or is it just like more weight?
it all depends! carbon fiber is lighter than standard bumpers so a carbon fiber body kit if designed properly will increase stability and handling and rear wings are a no no if its a FWD as it creates unwanted drag. but also you got to factor in your power to weight ratio in it aswell! like a full carbon fiber bodykit, bonnet, roof, doors, boot, and a simple strip will create a big weight differance and at the same time most carbon kits are tested properly. so some times it creates more weight, some times it wont
There are lots of bodykits around the real purpose of a bodykit is to reduce the lift introduced by the wind when driving at high speed... Now let us have a little detour.. imagine the plane's wing, the most basic shape of plane's wing is flat on bottom and curve on top. The practical effect of this shape is that when wind pass this shape, the wind above will flow faster and with lower pressure while the wind below the shape will flow slower and have higher pressure. so the bottom line is.. when the wind blows thru this shape it will actually have lift effect
and unfortunately, our cars' cross section are, in ways quite similar to the wing's. Let's say a car weight a ton and have 50/50 weight distribution, when the car's not running the weight pressing the front/rear axles will be 500kg each, right? But if the car is poorly design, when you drive fast the lift will cancel out major of the weight. Could you imagine driving said car at 250kph but the total weight pressing the tire to the road is only roughly the weight of the driver? Must be quite scary. That's when bodykits and wings come in to save your day. Wings installed on the top of your trunk will redirect air flow to create more negative lift, hence calling the lift force. Bodykits extended from your front and side bumper will lower the amount of air flowing thru underside of your car, also reduce lift force. (pardon the kph and kg stuffs, I'm more familiar with metrics)
Conclusion : - Good and well designed body kit will not add speed to your car, it will add to your car's stability so you are more confident to push more. - Badly designed body kits are dangerous, might as well reduce stability at high speed and might rice your car. :) PS. most of the effect of body kit will shown at the speed higher than 120kph (75mph)