So basically if you go over kerbs (i guess americans call it curbs) it boosts your speed. I personally didnt know about it until someone gave me this video, starts from 6:40

From what i understand it works in GTA IV too. Heres another video -

I havent tested how much it saves with random cars in single player but it does feel a bit faster. Who knows how much you can save it with untuned car etc. feel free to test.