Very soon we are going to be "moving" many of the custom vehicles into recognized vehicles. We are going to be methodical about it as we try to reduce the duplicate entries of vehicles that are really the same thing. We need to make sure that we don't create new vehicles which already have listings like your Fiat example.
I suspect your "national vehicle database" is purely made up of models sold in America. Thus in my case the Honda PCX 125(cc) Motorcycle is now "custom" whereas the Honda PCX150 is listed since it is a model built for North America.
This is a backwards step, when Fuelly has always had good support for Worldwide customer base. Heck, you even cope with the UK oddity that we buy fuel in litres but drive in miles.
Someone left their thinking hat off when making the decision to standardise vehicle models. I'd suggest you merge in the equivalent data from the registration/licencing/insurance bodies in the EU.
My 2014 Ford Fiesta ST has now disappeared as a model as well. I can no longer view other Fiesta ST users' statistics. It seems like 90% of the vehicle listings are now unassociated, unsearchable or orphaned.
This has happened to my motorcycle too. They seem to have omitted several models of motorcycle and probably cars too by the sound of it.
Mine is a 2013 Yamaha MT-09 and the MT-09 simply does not exist as an option or in the list of Yamaha models when you try find it in "Research vehicles" where the complete list is shown, so as well as being classed as a custom vehicle you now can't research these vehicles which have been omitted.
These vehicles were there before the update and now are missing. Bit of a bug that.