Hello. I bought a new 2018 Camaro SS in 2017 and have put less than 8,500 miles (mostly in town) on it since. It came with summer only tires, so I have all-seasons on it for about half the year. Last fall it developed a bad vibration at certain, but not all, speeds. The Chevy dealer changing out my tires said the rear all seasons were out of round and could not be balanced (they used road force balancer). I left them on and took it to a tire shop this spring. They confirmed a bad rear tire, but no one can explain why tires would suddenly go out of round after only 4,100 miles. It’s not flat spots because although the car has low miles I drive it regularly. The tire shop put on the OE summer tires and the vibration is not as bad and is not present at all the same speeds but is still present. I had noticed when the all seasons were on, pushing the brakes during vibration made the vibration worse, and the tire shop said they noted a brake vibration and recommended turning the rotors, which I’ve never heard of for a car with such low miles (even mostly city driving). So: What’s causing the vibration? Why would tires suddenly go bad with only 4,100 miles on them? Do I really need turned rotors after only 8,500 miles? Thanks.
Yes you have a bad tire… because sometimes tires go bad and because the car sits a lot. That is likley why the summer tires are lumpy after being remounted… if they were stored improperly that can happen.
Yes, you need to turn the rotors. The brake rotors get hard spots from sitting with the pads in one place all the time. The rotors get several hard spots from your infrequent trips and long spells of parking. This is worse with a performance car with big performance brakes. Get used turning the rotors or replacing them on a regular basis.
This is how brake rotors get hard spots.
Tester
This site disagrees… As do I. Car manufacturers have had this problem since they stopped using asbestos. The car sits, corrosion forms under the pad that is different than just rust. These spots don’t clean off as easily as surface rust on the rest of the rotor because the corrosion is different and harder than regular rust.
Nowhere does the article state that it causes hard spots.
However, if you don’t drive your car often or take long breaks, rust can sit too long, causing pitting in the rotor and requiring costly replacements.
Tester
Thanks all.
There seems to be common agreement about hard spots on the rotors causing a problem, even if the cause of the hard spots is up for debate.
Would “bad” rotors cause vibration at certain speeds, even if the brakes were not engaged at that time?
As noted, I have vibration even when the brakes are not engaged, that’s worse when they are?
Yes, they can.
Here’s an update. Nothing wrong with tires, wheels, suspension, or brakes. Was the A8 transmission shudder. NHTSA actually has a technical service bulletin that has been out since 18, that dealer never told me about! They say they check all bulletins whenever the car is serviced. Finally found a local, independent, mechanic (not technician), who told me about the TSB. Dealer changed out the transmission fluid under the power train warranty, and the vibration immediately disappeared. Cost me an almost new (4,500 miles) set of all season tires, though. When both the dealer and an independent tire shop told me they were bad, I threw them away. Just paid over $1,000 for my new set 
Is there another GM dealership that you could use–at least until the Powertrain Warranty runs out?
Yeah I switched dealers. New one did the flush.
I think you should contact the shop manager at the first dealer and tell him that his employees messed up. Unless he knows about their error, they will keep making it. He should be happy to listen to your issues as long as you treat it as an opportunity to help them avoid similar problems in the future. If he blows you off, it’s definitely them and you should never go there again.
Had problems with them before. This was the last straw.