You would have to be analyzing the hell out of your driving style and MPG's before and after to notice the difference. It wouldn't be very noticeable otherwise. The difference would be fairly small anyway. There are too many variables also to just base an answer off of what you have personally observed such as tire pressures, driving style, actual roads driven on, routes taken, ambient temperatures and so on.

I bet you probably haven't noticed increased stopping distances either, but if you were to test it with the stock rims and then again with your oversized rims in a controlled environment there would be a small difference.