A new study by Runzheimer International, an employee mobility company, says the costliest city in which to own a 2007 midsize car is Detroit, where it costs $12,210 a year to own a new midsize car. Following closely are Philadelphia at $11,081 and Los Angeles at $10,604. The No. 1 reason for the high costs was high insurance rates. Fuel, maintenance, tires, registration fees and depreciation were also factored in. The sample car used was a 2007 Ford Five Hundred sedan driven 15,000 miles a year.