Since July 2010 the U.S. is sanctioning companies which sell gasoline to Iran. Iran responded to the long announced sanctions by increasing its gasoline production capacity by some 50% and by lowering end user subsidies. Iran's production capacity will soon be 75 million liters per day while consumption has fallen by 13% and is now about 55 million liters per day. That leaves plenty of reserves and capacity for exports.

In May 2010 Iran still imported some 14 million liters per day. That fell to some 8 million liters per day in July after the U.S. sanction were activated, now Iran is a gasoline exporter. The U.S. gasoline sanctions against Iran failed!

As an exporter of gasoline Iran can now itself play the sanctions game and put pressure on the U.S. via its client state Afghanistan:

The price of fuel has risen sharply in parts of Afghanistan as an Iranian-imposed slowdown on tanker traffic at key border crossings has stretched into its second month, Afghan officials say.
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Hundreds of fuel tankers are stranded at the country’s main border crossings with Iran, stopped by Iranian border agents, and the number making it across has slowed to a trickle. About 40 tankers a day are crossing the border at Herat, Farah and Nimruz Provinces, compared with 250 to 330 a day before, according to commerce and customs officials.

Those 300 tankers per day would carry about 6 million liters per day. About 40% of Afghanistan's total civil fuel imports comes from Iran and it is mostly consumed in the south. Officially Iran is concerned that part of that fuel is going to U.S. forces in Afghanistan. It may indeed well be that some of that fuel is going to the U.S. military, but that amount would be too small an amount to be relevant.

According to a Deloitte study a deployed U.S. soldier is using up some 110 liters gasoline per day. Additionally there is one contractor for each soldier who will also need a lot of fuel. With over 100,000 men in Afghanistan, the U.S. military and its contractors must be using some 20,000,000 liters per day at a cost of $100 each (source (pdf)). One or two million liters skimmed off from the Iranian exports would not make a big operational difference (except for the dealers who manage that very lucrative transfer).

Iran will of course know this and I therefore believe that the real reason for this export ban is a different one. While the New York Times piece quoted above does not mention these at all (one wonders why), an earlier RFE/RL points to the more likely reasons:

Afghans affected by the resulting fuel shortages say they believe the crisis is Tehran's retribution for crippling sanctions imposed by the United Nations Security council this past summer. Analysts in Kabul link it to Iranian resentment over being left out of a multibillion-dollar gas-pipeline deal worked out among neighbors Afghanistan, Pakistan, India, and Tajikistan.

The TAPI pipeline, while now officially signed off on, is unlikely to be build as long as the conflicts in Afghanistan and the one between Pakistan and India are ongoing. For financial reasons it is dubious that it will ever be build at all.

That leaves retribution for U.S. sanction as the most likely motive:

[Kabul-based analyst Waheed] Mozhda says it is difficult to independently establish if any of the fuel may have been destined for use by NATO forces. But by closing the supply routes, he says, Tehran is clearly sending a signal to Washington in response to international sanctions imposed against the country over its controversial nuclear program.
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"This is another example of how regional and global rivalries affect Afghanistan and lead to Afghan suffering," he says.

Indeed the Afghans will suffer from these sanctions as they will have to get through the winter with heating fuel and gasoline prices increased by as much as 50%.

But the pressure will also be felt by the Afghan and U.S. governments which will be held responsible by the Afghan people. Resentment against the Karzai government will grow, U.S. sponsored economic projects will not realize any gains and resistance to the occupation will increase. The already stressed U.S. supply routes from the north and from Pakistan will now have to carry additional traffic for civil fuel to the south.

I expect that Iran will keep the slowdown of exports to Afghanistan on for a while. If only to make the point that it is relevent for any solution of the Afghan conflict. It may want to keep the sanctions up at least until the next round of nuclear talks on January 20. Progress in the talks could then be rewarded by lifting the export slowdown. If the U.S. sticks to its hardline position on the 'nuclear issue', Iranian fuel exports to Afghanistan could totally cease.

That would be very bad for the Afghan people. But as Iran has only a few non-military levers it can use against the United States, it may have to use this one to counter the sanctions against itself. As usually, Afghans be damned …