Wow, you really suck at math.
50 miles a day, * 7 days a week, * 52 weeks a year = 18200 miles a year
18200 / 15mpg = 1213 gallons of gas.
1213 * 3.08 = $3737 a year on gas.
So right there, you’re talking about spending more on a car then you spend all year in gas. Your math and logic = retarded, but lets keep going.
So you pay 5k for a BMW to “save money”. So you still drive the same amount, but for the next year you don’t even start the truck driving only the BMW, driving like an old lady, and getting 27 mpg. So you spend $2076 in gas during that year.
So you “saved” 1660 in gas. But you spent 5k to do it. So you’re 3340 in the hole. Yeah, retarded. But wait, there’s more.
You still have to keep insurance on the truck, since I’m guessing the bank owns it, and even if they don’t it’s not like you’re not going to be driving it at all. So now you’re registering and insuring 2 cars instead of one. You’re young, and I’ve seen your posts about tickets, so I’m guessing your insurance isn’t too cheap. Probably another 500 at least a year to add that 2nd car. So 550 or so for insurance + reg. No 100k+ BMW is going to run for a year without some parts costs, so lets just say another $500 there, being way overly generous on my side.
So now you’re about $4400 negative on your money saving venture. Remind me to never ask you for financial advice. 
BTW, the numbers look even worse if I use your 8-10 months instead of a year.