Re: very excited
So you live four miles from work, and a warmed up engine gets 11 MPG. You're using roughly 0.75 gallons of gas a day to commute.
If gas is $2.75 (it's higher than that here, but I don't know exactly where you are), that's about $2.06 a day, or $10.31 a week.
A gallon of premix is going to cost $2.75 + oil, figure $1.50 for oil. So $4.25/gallon. Now, let's be a little realistic and say your motorized bicycle will get 80MPG. (It also makes the math easier.) You'll be using 0.1 gallons of gas a day, which will cost you $0.43 each day, or $2.15 a week.
Let's add in the cost of the bike and some accessories, estimating about $250. We'll subtract the motorized bicycle fuel cost from the truck fuel cost, getting $1.63 a day difference. Dividing $250 by $1.63 gives us ... 153.37.
So with these numbers, you will get to the break even point after you have commuted to work the 154th time. That's just shy of 31 weeks. That's also not counting the cost of a helmet and jacket - both of which should be on your shopping list.
Of course, a cheaper build brings that down, as does higher mileage. Another thing you can do to really tip the scales in your favor is to drop your truck insurance from full coverage to liability (if that's possible for you).
Sorry, I was compelled to do the math. But I promise not to tell your wife.