Wooh, after looking through about four pages, I finally found this thread.
I'm trying to remeraber what episode this came from, but it's a scene where Homer and Lisa were in the car and they seem to be arguing about the radio music. And then Homer says something like, "When I'm driving, we'll listen to what I want. When you're driving, we'll listen to what you want."
And then lo and behold, Lisa is very proficient behind the wheel, listening to a song that annoys Homer. And then Homer wants to switch places again so he can have advantage of the radio.
I'm obviously way too late and I don't think it really matters but since it was never answered, the only episode I could think of is that Krusty Komeback Special (the one with the rival Gabbo) where the family helps Krusty lose weight after he gained so much from those low fat shakes.