I know FIRST HAND that my local Dodge dealer voided the warranty on a neon because the owner installed those ghetto looking SMALLER diameter wheels and tires. Claimed that it caused the engine to turn more RPMs at highway speed, so the powertrain warranty was denied. I actually took the time to argue with the zone rep about it and it was not my car. MY argument was, when you are towing, you are told in the owners manual to not use overdrive and that raises your rpms at highway speed. ALSO, what if he moved to a state that had a higher speed limit on the highway, that would cause the higher RPMs also. They would not budge, and I made a mental note to NEVER, EVER buy a Dodge or Chrysler product other than my Viper again.
Now, because of the magnuson-moss warranty act of 1975, it is AGAINST FEDERAL LAW to require that you buy tires, filters, etc from the manufacturer in order to keep your warranty in effect. Federal LAW states that if they require that, then they must supply them for FREE. This law is available for you to read online. The catch is, whatever you put on your car must *MEET OR EXCEDE THE ORIGINAL MANUFACTURER QUALITY*. That would be in effect for fluids, filters, brake pads, tires, etc. As for wheels, and lift kits, if they were able to argue like Dodge did that the extra RPMs at highway speed was putting extra stress on the powertrain, then you are screwed. Or, if they can show that the aftermarket wheels are heavier and caused suspention issues.....or bad offsets like in "the hood" caused wheel bearings to fail....But, even then, voiding the powertrain warranty if you still have a factory diameter tire height does not seem possible to me.
I sold my wifes Hyundai Sonata (2007) after we owned it for 1 1/2 years because I got sick and tired of them working their ass off trying to void her warranty for everything they could think of. We kept more detailed records on that car than any I have ever owned because I did not want to pay them insane prices for the non-stop required maintenance on that car, so I did it myself, and they threatened me every single time. I finally had enough and sold it. Will NEVER buy another.
I've been a GM man my whole life, but if they get militant about warranties, I won't put up with it from them either. I installed a head and cam package in my 1995 Z28 when it was 6 months old. The warranty file on that car shows I had them fix a power window switch and an ABS sensor. The broken rear ring and pinion, broken valve spring and everything else I broke from drag racing the car (at the track!), I paid for the parts and installed myself. I"m not out to screw anyone. BUT, if I install an aftermarket catback of better quality than the GMPP one and they try to void my warranty on my new Sky Redline, there will be pure hell to pay. The aftermarket catback will not put ANY more stress on the powertrain than the one GM sells, so that is BS.
GM protecting itself from abuse is one thing. Screwing CUSTOMERS who have bought your product is something else completely. This guy having installed exhaust headers had absolutely *nothing* to do with a lose steering bolt.