Are 205/55R16 and 225/50R16 Tires Comparable? - Wheels and Tires Message Board

I recently took my car to get new tires. I took the salesman at his word and selected some new tires and had them installed. After driving the car for a day, I noticed that at high speeds (greater than 40MPH), I can feel the pedals vibrate and hear a loud humming noise (almost sounds as loud as the engine). When I turn the wheel to the left, the noise subsides. As soon as the wheel returns to the normal position, the noise comes back. The noise becomes louder and more high-pitched as the speed increases.

I inspected the tires, and to my surprise, they are a different size than what my car calls for. I have a 1996 Audi A4 2.8L Quattro and had the factory-specified size of 205/55R16. The tires that are now installed are 225/50R16.

I took the car back to the company that sold the tires to me. I got mad at the owner for installing a tire size that was different than what was supposed to be on my car. He was defensive and unhelpful, and told me that the 225/50R16's were almost identical to the 205/55R16's and that wasn't the problem. He said that it was a problem with my car- possibly a bad bearing or some other problem.

I understand that new tires will sound different than the old ones, however the noise doesn't seem normal. It seems to be coming from only one tire (front driver's side) and produces an abnormal vibration and loud noise. Are these two tires in fact comparable? What could be causing the noise? Could the problem actually be my car and not the new tires? I am planning on buying different tires and taking these back to the shop and having my credit card company issue a chargeback, but I want to make sure that I am justified before I do that.

Thanks for any help!