Disclaimer: Car has no shakes or vibrations at all with original Bullitt wheels and 245/45/17 all around.

So, I took my 17x9 wheels and had 275/40/17 tires mounted on them. Had wheels and tires "spin balanced" on the machine. I watched them do it and there are weights on 3 of the 4 wheels.

Brought them home and put them on car. Car vibrates in steering wheel and seat. I only drove it about 100 miles total so far. It seems like it's getting better or I'm getting used to it. I'm going to switch the fronts and backs and see if that helps.

Question is: Is spin balancing a really complicated thing. I just assumed if you stuck them on that machine and put the weight where it told you, it was gold. I've never really had this problem before. Is it something the shop did wrong or do the 275/40s feel/handle/wobble or do something different on the front that's causing my vibration?

I don't want to be a real A-hole and take my car back and have them check them. And... I REALLY REALLY REALLY don't like air wrenches on my lugs. I have lots of bad luck with that.

Thanks for any input!