In the reversal of a 10-quarter-long trend, new vehicles are now more expense to purchase than they were earlier this year in inflation-adjusted terms. According to the Auto Affordability Index compiled by Detroit-based Comerica Bank, the purchase of an average-priced new vehicle during the third quarter of this year required 23.2 weeks of median family income, before taxes. This compares with 23.0 weeks of income required for purchase in the second quarter.

Though the news was a bit troubling, Comerica Bank did note that during the same quarter a year earlier, a new vehicle purchase took 24.4 weeks of income, more than a week’s worth of income more than this year.

The average vehicle price in the third quarter was $21,804, up $118 from second quarter, but down $657, or 2.9 percent from the same quarter a year ago. Though prices are up slightly, the real culprit seemed to be interest rates.

"Higher borrowing costs played a major role in reversing the 10-quarter improvements," said David Littmann, chief economist with Comerica Bank. "Average rates rose more than 0.5 percent in the third quarter, up nearly a full percentage point from a year ago, and at the highest point in three years."

Comerica’s Auto Affordability Index is compiled from Commerce Department and Federal Reserve data. It does not involve the use of a dartboard or Tarot cards.