Why conventional car-buying wisdom is costly and wrong

When it comes to buying a car, conventional wisdom couldn't be more wrong. While consumer advocates have been telling people to buy cars at month's end to snap up the best deals, sales data shows that the last two days of the month are actually the most costly time to purchase an automobile.

Think prices are best in December when dealers must make room for new models? That actually happens in August. In reality, prices are considerably higher at year-end.

"When you look at the real data, you see that you're almost always better off doing the opposite of what conventional wisdom tells you to do," says Scott Painter, chief executive of TrueCar.com.

TrueCar, an auto shopping site, has been collecting car sales data for five years and recently crunched the numbers to determine the best days, weeks and months to buy. The results were surprising.

Consumers have long been told that they should make car deals at month's end because that's when dealers will be scrambling to make their sales quotas, but the best deals are actually struck on the first two days of the month, says Painter. Consumers save an average of $390 by striking deals on those days, he says. The most expensive time to buy a car is on the last day or two of the month, he adds, though the final week of the month (aside from those two days) is a pretty good time to shop.

Think you'll gain an edge if you visit the dealership during the workweek? Nope. You'll actually pay vastly more. The average price of a car sold on Friday is $31,351 -- $2,051 more than the average price of cars sold on Sunday, $29,300.

Of course some shopping tips are intuitive. For instance, when gas prices peak, so does the cost of fuel-efficient economy cars. Smart buyers should snap up those cars when gas prices dip and everyone else is looking at SUVs. Likewise, if you're looking for a gas-guzzler, go to the dealership when gas prices spike and SUV dealers are likely to feel lonely and demoralized.

You should buy exotic sports cars in January, Painter adds. Why? Their highly-powered rear-wheel drive engines perform badly in wet and freezing weather, so sales slump in the winter months, says Painter.

"Prior to having data we had conventional wisdom," says Painter. "Unfortunately, much of the conventional wisdom is just wrong."