Note from a Tehrani friend

Iran has a lot of crude oil. But the country doesn’t have enough refineries to process this oil into petrol. Iran sells its crude oil to countries abroad where they have refineries to process crude oil. Iran thus buys petrol from these countries for a high price.
But the price of petrol sold in Iran is 1000 Rials/Litre (8 European cents). The government won’t be able to sell the petrol for the same price they purchase as people won’t be able to afford it.
The black market took advantage of this. Petrol trucks were filled up and the petrol was sold to neighbouring countries for a good price on cost of the government i.e. the tax payers. To prevent this, the government introduced the petrol cards.
Every car receives a petrol card. This card is limited to 100 litres of petrol per month. When you drive to the petrol stations you first insert your petrol card. Then you pay for the amount of petrol you received. Every time you refuel, the amount of petrol you bought will be subtracted from your card. If the 100 litres are used before the end of the month you won’t be able to refuel before the beginning of the following month.
Another version is that you receive a card for 600 litres/6 months. If you are not careful and use the 600 litres before the end of the six months period you have to wait for a long time before you receive your new card.
Four months ago the amount you received was raised by 20 litres. The government announced that from this year (the Iranian year starts on the 21. March, with the beginning of spring) people can get more petrol if the 120 litres aren’t sufficient. But they will then have to pay 4000 Rials (32 European cents) per litre.
Although the introduction of the petrol cards was to prevent smuggling, the general impression is that the government wants to prepare people for coming economical sanctions.