If your in a hurry and you have the auger you can attack the least useful cars (the ones with just the chassis) and get only Iron and Gas and in the wasteland there are so many, you can attack 30 of them and get a lot of iron and gas very quickly, although leave the undamaged and the half damaged cars alone for a wrench later on.
I too scrap a lot of things from springs to rebar frames, if you find rebar and you do not need them, they put out a lot of scrap. I have also been scrapping every canned food other than Salmon, Chili and Dog Food for obvious reasons. Tools will also give a lot of scrap, like sledgehammers, wrenches and pickaxes. This is based on my current game in A17.4, I cannot even imagine what the changes are so I really cannot give much advice on the new alpha.
Like said above, scrap, wood and stone are always in my backpack at the start of a game at the top row after dukes, later in the game I keep some Forged Iron and Steel next to it.
You know this one thread of all the threads makes me want to get back in the game today. I haven't been into it the last week or so, so thank you for this motivation to get back into it - at the moment I am 1 day away from laying my concrete base floor 60X60 Rebar Frames and Concrete.
55 Concrete Buckets 55000 Total Concrete
600 Spike Traps
150 Iron Bars Bottom
100 Iron Bars Top
11000 Rebar Frames 22 Stacks of 500
100 Concrete Plates
I have all but about 1000 more rebar frames.