I have been thinking about replacing my truck with a custom 3-wheeler, like the Vortex or the RQ Riley Xr3. After the initial ‘start up’ cost, I could save a TON of money on fuel, probably on insurance and licensing too. It would technically be registered as a motorcycle I think, but maybe a ‘prototype’, not sure.
Madjak is right that the best way to save money (and depending on what your drive, only way) is to replace your vehicle with the bike. How ever, the way that I justify it is that it saves me on my month-to-month expenses (mostly fuel), which allows me to pay everything off faster
Jeff also has a good point, that for about the same amount of money you can get a sub-compact that gets about the same MPG. Not near as fun to drive, however.