I recall the question was prompted by a front set of Dunlop's only lasting 15k on a Toyota Verso. The subsequent Michelins lasted longer.

Seems to me it changes over time. Probably down to availability and price.

I'd have thought car makers would contract for (say) 50,000 tyres from the best bidder, working in the same way as fleets to extort a good deal. I doubt that every example of a particular model would hit the road with identical tyres.

For a car manufacturer in the volume market 50,000 tyres would be enough to supply just one production line for typically less than 4 weeks. OE tyres are required in vast numbers, delivered to car production lines on a "just in time" basis, with no breaks in supply. It is therefore very good business for tyre manufacturers and the needs of the car manufacturers overrides those of the aftermarket. The volumes necessary dictate that more than one tyre manufacturer will be chosen to supply and the logistics of getting the tyres to the lines in the required numbers at the right time are very important to the contract decision.

Price is also a factor and it will obviously be much lower than retail. With high volumes and an assured continuous demand the tyre manufacturer can cut margins and, since they deliver direct to the car manufacturer, there are no wholesalers and retailers adding their profits on top.

OE tyres are usually specifically tailored to the application and undergo a lengthy development and approval process with the tyre manufacturer's engineers working with the car manufacturers engineer's to arrive at a specification which passes all the tests and can be signed off for production. Usually a number of different tyre sizes and types will be needed for a car model in its various specifications and all of these will need to be approved separately. The costs involved are huge so the car manufacturer will only do this with a few previously selected tyre suppliers. For very low volume car models, only one tyre manufacturer may be chosen prior to design and development of the vehicle due to the costs involved. Historically, the only OE specification tyres clearly marked for specific vehicle fitments were those with characteristics critical to performance (typically for rear or mid-engined sports cars), but recently this practice has been adopted more widely by car makers. Alternatively an OE tyre specification can be identified by the relevant product code (for those "in the know")

There is an obvious conclusion to the question asked. A mainstream car maker does not buy tyres on an "ad hoc" basis based on price and availability. The constraints imposed by car design/development/pre-production testing and meeting the requirements of legislation, followed by just in time supplies to a production line mean that tyre suppliers are chosen at the beginning of the process. The decisions will be based on a tyre maker's ability to provide all that is required and contracts are drawn up accordingly. With such high costs involved in the approval process plus the demands of legislation, it is rare that alternative tyres would be considered once a car is in production until there is a significant model change or facelift.