The relevant costs involved would include unit production costs

The relevant costs involved would include unit production costs (depending on machinery used), insurance premiums compared to the irrelevant costs of fuel and individual motor parts. Sunk costs here could include the plant & equipment used in production together with the factory and general production site & transport vehicles that all would have already been purchased and so would need to be liquidated.

I would use differential analysis to assess whether, in the financial model for outsourcing the manufacturing activities to a vendor, the operating income earned would outweigh that from keeping the production in-house (as opposed to looking at the entire income statement for each scenario). 

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