Mike,
Sorry, but please take a good look at that crow on the branch outside your window. You are about to become very familiar with it.
Long before the first vehicle comes off the assembly line, vehicle manufacturers put out tire specifications for every vehicle they produce. ONLY if a tire passes all those specs (which can take as long as 2 years to go over all the hurdles) is it allowed to be supplied to that assembly plant. If more than one tire manufacturer passes all the specs, then there might be a single tire supplied, or multiple manufacturers.
If there are different sizes on a given vehicle, then the specifications may be a little different or a lot different, but there will be specifications that have to be met.
Some specifications are very difficult to get the compromise right, and some specifications are easy.
Almost without exception, the tires supplied to vehicle manufacturers are different than what a tire manufacturer would make for the replacement market - and typically the issue is the compromise between rolling resistance (which relates to fuel consumption), and how it affects traction and wear - which is why you’ll hear a lot of complaints about the wear and tractions qualities of tires on new cars. It isn’t that the tire “quality” is less - it’s that the compromise dictated by the vehicle manufacturer’s specs is biased towards fuel economy.
So in some respects tires ARE custom made for the vehicle manufacturers. They are supplied in HUGE quanities - as high as a million a year! - which is why tire manufacturers agree to make such oddball tires. Who wouldn’t want to supply that many tires, shipped to the same location on a steady basis, for 3 years straight? Sure makes setting up an assembly line easy!
One of the ways tire manufacturers handle this is by creating lines of tires that are specifically designated as OE - since those tires might have some performance issues that would “taint” the whole line. For example: Goodyear produces a line of tires called “Integrity” that are commonly used as OE. EACH tire size is designed to different specs - according to which vehicle that tire is being supplied to.
But vehicle manufacturers are a pretty smart group and they know that if there is a supply problem with one tire manufacturer, then they’d better have a back up in place - which is why you may see different tires (of the same size) supplied. Nevertheless, all those tires had to meet the same specs. Sure, there will be differences between brands, but the differences will be minor compared to the specs.
BTW, run down to your local Chevrolet dealer and look at all the tires. They all have a TPC number branded on the sidewall, indicating they a Tire Performance Criteria with the specification number that follows those three letters. You will find a TPC number on ALL GM tires - and if you know what to look for, you will find such designations on Fords, BMW’s, Mercedes, Jaguar, Porsche’s, etc.