I remember back when I used to ride a bicycle (man that was YEARS ago; that's got to change) I noticed that the mountain bikes felt like they had more rolling resistance than the cruisning bikes with their skinny tires. This made me wonder if getting skinny 'donut' tires on my car would increase fuel economy.
I purposefully put the smaller tires on the rear so that my speedometer and gearing would remain the same as with the larger tires.
These are the documented results:
I should add that I don't use these tires on a regular basis because of their lack of traction, ride quality, and quietness.
Feel free to discuss.