For the FG we found a place, forgot the name Oddizer or something like that. Need a LOT of $$$ im guessing.
Nitro, no idea, and Nitrage, I heavily doubt it. On the other hand, 'soup' it up your self.
IMO Offroad + Speed dosent match well, Even the best drivers dont get they're truck going 70+ MPH.... They focous more on handling.
Its like Hamann for a Range Rover and Gemballa for a Porsche. They NEVER focous just on Speed and HP ratings, they focous on handling

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*** Edit : Deleated Pic.***