But about 10 million drivers break these rules every year, according to the Internal Revenue Service. More than 60 percent of the 16 million taxpayers who claim business-related auto expenses each year do so inaccurately. Some errors benefit the government, as when taxpayers mistakenly classify business-related expenses as personal ones. But the net result is a loss of about $6 billion in annual tax revenue.

These losses have been piling up for decades. In 1984, Congress took measures to curb them, including requiring taxpayers to maintain written records of their business-related auto use. But just one year later, after an outcry over the onerousness of the new paperwork requirements, most of them were repealed. Since then, politicians have turned a blind eye to the problem.

Self-employed small-business owners are frequent violators, but not the only ones. One case settled in 2011 involved a professor of criminal justice at Rowan University who claimed to have driven 18,000 miles “on business” and took more than $7,000 of auto-related deductions. In fact, the government asserted that his only business expenses were eight trips between Rowan and Temple University, less than 25 miles away, and denied the deductions. In a 2009 case, a sales consultant in Miami claimed more than $10,000 in auto-related business expenses, but when the I.R.S. challenged the deductions, she could not offer any concrete evidence for her claims. (The court ruled against her.)

However egregious the cases, these violators were the exception only in that they were caught. It took costly I.R.S. audits and court proceedings to catch them, and since only 1 percent of taxpayers are audited, many slip through the cracks. With salary payments and bank interest earnings, third parties like employers, banks and brokerage firms issue “information returns” (through W-2 and 1099 forms, for instance) can corroborate taxpayers’ reports on their returns. But there is no outside party available to monitor whether a taxpayer’s automobile use is business or personal in nature.

Even worse, the tax deduction for gasoline creates bad incentives. It encourages violators to buy less fuel-efficient vehicles, to live farther from their workplaces than they otherwise would, and to take longer and more frequent leisure trips. All of this not only harms our environment and deepens our dependence on foreign oil but also increases the price of gasoline. Indeed, we estimate that, based on current levels of demand and supply, the added demand by noncompliant taxpayers drives up average gasoline prices by an estimated 5 to 10 cents a gallon, at least in the short run.