How in the world do you figure $0.50 per mile?
Even if you assume a modest 20 mi/gallon at $3/gal, you're looking at about $0.15 a mile. A $40 oil change every 5000 miles, and a more expensive maintenance job every 10000 miles isn't going to push you past $0.25/mi.
Are you amortizing the cost of the vehicle?