I can't be sure about this but I think you have to unlock the cars in the Ridge State GP before they can be used online. The online and offline modes have the same effect on the player in RR7, Winning online improves your credits and fame points in the same way that winning offline does. If the room is using arcade machines then you should be able to pick any of the cars from a class but if the room is using player machines, you can only use machines you have unlocked and modified offline.
The fierra is the basic car that's given to everyone regardless of how they're doing offline but it won't be anywhere near as good as the cars that others have unlocked and modified to their own preferences.
This is also the reason why the UK racers aren't doing too well, They're racing online without playing enough of the offline mode, meaning they don't have many cars to choose from (fierra) and they're also not as experienced as people who have completed a lot of the offline content who would have more cars to choose from and also be much more experienced in racing in those cars.
One thing I've noticed with player from everywhere is that hardly anyone seems to be using dynamic drifting cars, it may be that I haven't played enough online but I just find it strange that more people aren't using them. My car of choice is the Raggio and I've only played one race when someone else was using one as well and only 3 when anyone else was using a dynamic car. That was out of 15 different races.
I've done 63%...done all the manufactuers (I think) won RGP 13 all the Single Even Races before all the new ones became available after completing RGP13 and have bought most of the cars as I have so much credit and modified them
Yet I still get the Feira, I feel like I'm being scr3wed here!