Uncle Sam has devised a tempting tax break for American consumers who patrol the nation’s malls, stockpile frequent-flier miles or spend beyond their means. They can deduct the sales taxes they rang up last year instead of their state income taxes.

The choice – which is available only for 2004 and 2005 unless lawmakers extend it – is a no-brainer for residents who live in Nevada, Florida and five other states that don’t charge income tax. For the first time since 1986, they get to deduct one of their biggest forms of state tax.

But it could also be a boon for some taxpayers in states like Illinois, where the income and sales tax rates are roughly comparable. Even in states with high income taxes like California and New York, many people will be asking the same question: Am I one of the chosen few?

That possibility is likely to be the talk of the tax season – and that has tax preparers sweating. They fear they’ll waste hours answering questions and crunching numbers for clients who won’t benefit.

“That’s going to be one of the most time-consuming problems of the tax season,” said Robert Duitz, who owns RD’s Tax & Financial Strategies in Campbell, Calif. “It’s not a deduction we want to miss … but we have to go through the pain and suffering to see whether it makes sense or not.”

At first, tax experts figured residents in high-income-tax states generally would save only if they had bought big-ticket items such as cars, sport-utility vehicles, RVs, boats and airplanes. But as tax experts studied the rules, other potential candidates emerged, including:

-Retirees who owe little or no income tax on Social Security payments, interest from Treasury bills and municipal bonds and certain types of annuities and pensions.

-Do-it-yourself home remodelers who spent thousands fixing up their houses. Homeowners who hired contractors might miss out if the contractor paid the sales tax, however.

-Self-employed people who owed little income tax because they had a bad year.

-Large families who stack up tax-saving exemptions for their kids.

-Layoff victims who saw their incomes plunge.

-Disabled workers who survived on nontaxable state disability and workers’ compensation checks.

“I think this will apply to a lot more people than they think,” said Claudia Hill, who owns a Cupertino, Calif., tax-preparation firm and is editor in chief of the Journal of Tax Practice & Procedures.

It can be difficult to divine whether you’ll come out ahead without crunching the numbers. That’s especially complicated this year because nobody knew to save receipts before the Jobs Act was signed into law Oct. 22.

As a result, most taxpayers must turn to the Internal Revenue Service’s optional sales tax tables in Publication 600. That will lead to the start of jerry-rigged calculations that could make even Rube Goldberg snap his pencil in frustration.

The tables offer state-by-state breakdowns of how much state sales tax taxpayers presumably paid, adjusting for income and the number of exemptions the taxpayer claims. Beware: You’re likely to shortchange yourself if you rely solely on the IRS tables.

The tables don’t include local taxes that can substantially boost the amount you can deduct. For example, the New York state rate is 4.25 percent, but New York City residents can deduct more than twice that figure because they pay 8.675 percent when local taxes are added on.

“The table is less than half what you’re entitled to,” said Martin Nissenbaum, a national director for Ernst & Young. “Unfortunately, someone might pick that up and say, “I’m not going to bother.”

After accounting for local sales taxes, then you can lump in sales tax on other big-ticket items you bought, like a car. If you moved between counties or moved into the state during the year, prorate the taxes based on the number of days you lived in each locality.

“It’s incredibly painful for something you would have thought would be truly simple,” said Kathy Burlison, director of tax implementation for H&R Block Tax Services.

Once you calculate the sales tax total you can start making the big decision: Should you deduct your sales tax instead of your income tax? Rules of thumb can be risky, but here’s a framework:

-If your sales tax is higher, take it.

-If your sales tax is a lot lower, skip it.

-If your sales tax total is close, give it more scrutiny if you’re expecting a state refund on your 2004 taxes. When you claim a deduction for state income tax, state refunds generally are taxed the following year. Part of that refund may escape tax, though, if your itemized deductions were only slightly higher than the standard deduction.

If you deduct the sales tax instead, you don’t have to report your state income tax refund on your 2005 return. Avoiding the tax on the refund might save you more next year than you’d save this year by deducting the income tax.

Before you work your pencil to a nub, keep in mind one fact: The vast majority of people will get no reward for puzzling over this.

“In reality, that’s good academic cannon fodder,” said Daniel D. Morris, a partner with Morris + D’Angelo, a CPA firm in San Jose, Calif. “To the average traditional taxpayer, the mental anguish associated with those kinds of calculations far exceed the benefit. It’s not worth the effort. Go out and play with your kids or walk the dog.”