there are several speed boosts in lfs.
first of all buying a S2 license gives you an enormous speed boost, because it has turbo cars.
also when you are right behind someone you have somekind of boost, its called slipstreaming.
or you can download LFStweak, and give your car 10 gazillion horsepower which will give you a good amount of speed boost.
or when you are really desperate, then get the NOSz cheat and that will give you an unrealistic amount of speedboost and a ban.