In 1932, Dr. J. A. Purves invented the Dynasphere, a ten-foot, thousand-pound wheel of steel that didn't exactly revolutionize personal motoring. The original 2.5 horsepower gas motor could achieve a top speed of about 25 miles per hour, with the driver sitting in the middle, tipping the thing left or right to steer...and clenching his teeth, hoping nothing was in the way.
You may ask: why does the name reference a sphere, when this thing looks like a wheel or a hoop? Well, the outer body of the Dynasphere is actually a portion of a sphere -- it's what you'd get if you chopped the sides off of a big metal ball. I haven't found a good reference as to why the Dynasphere failed to take off, aside from the obvious driver visibility issues and one mention of a pedestrian being hit during a test drive. According to various online accounts, there were multiple prototypes produced, including an electric version. Here's newsreel footage of the vehicle in action:
You can read a bit more about this weird thing in Popular Science. I also found a bit more test footage with an inappropriately modern soundtrack, and a nice collection of images (including other crazy-big-wheel vehicles, and the delicious quote: "Dr. Purves claimed that the use of one wheel instead of four gave great economy of power. This seems highly doubtful.").