Tire Switcheroo

So I had Goodyear Integrity tires on the car when I bought it in '07, and got a bit over 40,000 miles on them, with an average of about 48 mpg fuel economy. Switched them (no idea why) to Pirellis and immediately dropped to around 43 mpg. Finally, after 50,000 miles these wore down, and I swore I would go the other way, getting tires specifically made for fuel economy. Here's the problem.

Ordered Goodyear Fuel Max tires, and had my wife go to the dealer to have them installed. Then we go out for a road trip and, 1,000 miles in, I realize I don't have Goodyear tires on my car-- they're Michelin Harmonys. Wha? I look at my receipt: Goodyear. I look at my tire: Michelin.

Okay, so the Michelins are rated at lasting 15,000 miles longer, and were about $20 more expensive per tire, so I got the better end of the deal... but what about the fuel economy? Anyone have any experience with both of these tires? Is it worth going back to the dealer to have them switch out these "better" tires for the ones I paid for, or should I just go back and make sure they amend my receipt for warranty purposes?

Thoughts?