The Great Lakes. I lived in Ohio and Michigan.
Cheating!
Does remind me of an epic ride I did in the 80's with some buddies though. We had something here called the "Four Corners" where you visited each compass point on mainland Britain. To spice it up, we added the two furthest points apart by road (to be done non-stop) and did it all for the MS Charity here.
If you ask the average Brit to name the furthest point E, W, N & S I'd be amazed if they could get more than 2 right. To save you Googling it, in order it's Lowestoft, Ardnamurchan Point, & Dunnet Head Lizard Point. Furthest apart by road? Land's End to John O' Groats the traditional distance by road being 874 miles. That leg took us just a few minutes over 20 hours if memory serves me well.