1918 Smith motor wheel, model C buckboard

 there is no accelerator pedal, just a brake pedal. To go at different speeds you raised and lowered the motor wheel. It went at a constant speed, and if you didn't want to go that fast you raised it occasionally to get something between stop and full speed.

for all you might want to know, or see about the Smith, which ran from 1915 to 1919 and then was bought by Briggs and Stratton, see http://justacarguy.blogspot.com/2010/01/briggs-and-stratton-flying-buckboard.html and for an engineering drawing http://justacarguy.blogspot.com/2011/07/4-wheel-buckboard-with-smith-motor.html 

Monowheel circa 1952, photos by Dean Loomis. WW2 was just over, and anything was possible

These were all photographed for Life magazine, as this was being prepped to get to a motorshow in 1952. Sleuths who try to figure out where, would need to look for a Bob's Speedo Service and a Hancock gas station in 1952.

Found on http://interweb3000.blogspot.com/2012/02/vintage-monowheel.html
via http://thenewcaferacersociety.blogspot.com

The monowheels in Men In Black 3 look a lot like this one http://justacarguy.blogspot.com/2012/03/men-in-black-3-mib3-is-going-to-be.html

Men In Black 3 ( MIB3 ) is going to be another great MIB movie, and in this one, J and K ride motor wheels!

 Looks like they are strored under and inside the 66 Ford (Fairlane?) for quick deployment

I felt the Steampunk desire today, So I watched "Steamboy" and grabbed these screen shots, cool movie, just make sure to select the language options of your native tongue and avoid reading the whole thing

 any movie with a motorwheel can't be bad. A steam powered one is even better.

 Good kid has a runn in with the bad guys trying to steal the plans and prototype his inventor dad and grand dad sent him, and the kid tries to escape, using that motorwheel, gets chased to a locomotive, which gets attacked by a blimp ...

 later the bad guys send out an army of armored soldiers, and this aeronaut

who all are engaging in a war with the british who have these cool old tanks. The time frame is Victorian, and before any vehicles but the trains and tanks.