http://wiki.polycount.com/PhotoshopTools?highlight=%28%5CbCategoryTools%5Cb%29#Export_Tools a whole lot of them on the wiki. I personally like d1vSave, dunno why :)
This could be useful to you: http://wiki.polycount.com/TexturingTutorials?highlight=%28\bCategoryTextureTechnique\b%29 or even just browsing around the WIKI - http://wiki.polycount.com/Polycount
keep in mind that it's vertex count that matters. Here's the bit from the wiki. http://wiki.polycount.com/PolygonCount?highlight=%28%5CbCategoryEnvironmentModeling%5Cb%29#Triangle_Count_vs._Vertex_Count
IGN interview, mostly about the coming DLC and the much needed PUNISH THE RAGEERS! YUS! http://uk.ign.com/videos/2015/07/28/the-future-of-rocket-league
Check your Projection Cage and padding. You also might want to have a look at this http://wiki.polycount.com/NormalMap?highlight=%28%5CbCategoryTexturing%5Cb%29#Sloped_Extrusions