Last week I got my car serviced at the dealership (a thing I only do because it's mandated by the company, because only they know how to turn off the computerized reminder and calibrate all the stuff, and because it maintains the resale value-- that said, I always ask for a discount, and I always get it).
I had to leave my car there overnight so I walked home, and the next morning I got that totally ridiculous call "Hey, your rear tires are worn out--- do you want us to put on new ones? It's only (insert outrageous number here)." I was like "no thanks-- I'll get them at Costco," and from the shock in his voice I'm going to guess that alot of customers just take his word for it and pay the higher dealership price.
So-- the moral of this story is, I got the tire size from him, looked around on the internet, called Costco and Walmart, and finally found the lowest price at Costco. They had to special-order them and I'm getting them put on this weekend, when I will also be taking in several price comparisons I did while I was waiting, just to see if they can give me further discounts.
I know, this sounds tedious, but the net savings was over $300, so I would consider that totally worth it.