I don't think cars would 'ruin' the series, the game just needs to be properly designed for cars to be implemented correctly.
Like the map, it needs to be bigger, waaay bigger. If not then we will be able to go from one side of the map to the other in a minute.
What would ruin Fallout would be to turn it into a racing game with stunt jumps and crap like that like it's some kinda action game.
But cars themselves? Doesn't ruin Fallout at all. Fallout 2 had a car and it worked just fine. Tactics had vehicles and while they handled a bit iffy in the local maps they worked well otherwise and FOnline's cars also work well.
Now multiplayer on the other hand could very well ruin Fallout as it is a slippery slope and they might change too much of the singleplayer experience so that the multiplayer experience can work, or budget is wasted on a feature that isn't really worth a damn, or the save system is changed to accomodate coop multiplayer on consoles.