The cars / motorcycles industry is a fun industry to play with, while it shares similar characteristics as other products, selling Cars & Motorcycles can provide very good returns and gives you exposure to most areas of the game. I've gone through and flow charted all of the lines you can take to get to manufacturing. Just a few notes about the product lines before viewing the flow charts, a lot of these goods can be produced / used internally within your manufacturing infrastructure. You can also control quality entirely for all tiers of manufacturing if you desire, it will cost more to do this (general OpEx + R&D) but it will allow for greater control for quality.

*As you can see, both products require engines and Wheel/Tires (x4 for cars x2 for motorcycle). The only difference is that cars require car body whereas motorcycle just requires steel. The required quantity of input is noted below each product.

*These flow charts show the T1-3 products required for the final T4. Rubber, plastic, and glass aren't a real necessity to be manufactured internally. I would prefer to manufacture steel over all pre-requisite products as it is a main input for all tiers.
While I highly recommend maintaining internal sales for T4, you can generate additional revenue streams by allowing selling off any pre-req products (ie; steel/plastics etc) depending on how far down you choose to manufacture. Only do this in a conservative manner, for selling high quality T3 products used for cars/motorcycles can possibly give your competitors an advantage or even the playing fields. I personally will only sell off steel I don't need. Glass/car body/engines/wheels & tires i keep internally.
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At the start of the game, you can start to manufacture the T4 goods (cars/motorcycles) by using imported goods. A small to medium sized factory is usually suffice for several autodealerships until the economy has developed enough for you to expand. I have observed that eventually you will require several factories per city to keep up with the demand for sales in most situations. Using a warehousing system to store T3 inputs (engines/carbody/wheel&tires) to help bolster rapid supply & demand changes.
Margins are very minimal in the early game, once you have better quality and higher technology you will have to pay attention to pricing and fight a thin margin battle.
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To be continued.