Secret tips for buying a new car

NEW YORK (Fortune) -- Want to know the best day to buy a car next month? It's April 12th, the Monday before taxes are due, according to a new study. Since shoppers will be preoccupied and feeling poor, dealers will be eager to make a deal so they can move some inventory.

The worst day will be likely Easter Sunday, April 4th. Shoppers will be out in abundance and eager to celebrate the holiday by buying with a new model. Dealers can sit back and just take orders.

Which cars carry the biggest discounts as a percentage of sticker price? The survey says two of them are Chevies: the Cobalt and the Malibu both sport $3,000 incentives. But there is also $6,000 on the hood of the Volvo XC70, and a whopping $10,000 for the customer who can afford the $80,000 BMW 6 Series.

This kind of data has long been difficult to collect and, when it is, rarely disseminated. It is the rawest kind of competitive information and it can dictate the shape of marketing and incentive campaigns costing hundreds of millions of dollars

A new service called TrueCar says it has acquired transaction information for more than 43% of all the new cars and trucks sold in the U.S. For the first two months of the year, that represented more than 600,000 sales.

TrueCar has massaged the data to show which vehicles sell for the greatest variability from the sticker price. That lets analysts know which models are resonating in the marketplace -- and informs the customer where the best deals are.

Want to know which brands are offering the largest incentives? TrueCar ranks Ford and Hyundai first and third. Ford is offering discounts that are 13% off the sticker price and Hyundai 12%. These two makes have been heralded for their sales results this year, but, if the data is accurate, their success may be coming at the expense of profits. Chrysler and Jeep rank second and fifth, which is no surprise, because both are short of new models, and their dealers have been in revolt.

Some of the TrueCar data requires an extra layer of analysis. For instance, the service lists five 2010 models with the largest discounts below sticker price. Four of them, however, are being replaced this year or are being phased out -- the Hyundai Sonata, Jeep Grand Cherokee, Jeep Commander, and Chevrolet Cobalt -- which means they are being discounted more heavily to clear out inventory in the path of new models.

The same goes for the list of cars with the least flexibility in their transaction prices. Topping the list is the Mercury Grand Marquis, an old-fashioned body-on-frame sedan that hasn't been updated since the first Bush administration. The Grand Marquis is in short supply and is sold primarily to older buyers, who are presumably set in their ways and aren't inclined to dicker over price.

Likewise the Mazda MX-5 Miata and Chevrolet Corvette are listed as among the models with the most price flexibility. That presumably will change when spring arrives and demand surges for convertibles and sports cars.

Still there is a boatload of data here that provides new insights:

Which manufacturer does the best job of managing its end of the year sell-down? Subauru, where only 0.3% of its sales came from 2009 models.

Which vehicles move fastest off the lot? The Mercedes GL550, BMW 335D and Chevy Equinox. Blame limited production for the two German makes and a hot new model for the Equinox.

Which sit around the longest? Among the top five are the Ford Mustang, Chevy Malibu and Dodge Grand Caravan.

Sometimes reports like these have the effect of altering consumer behavior. So if an army of shoppers hit the showrooms on April 12 looking for a deal, the discounts will dry up.

Car buying just got a bit more interesting.