Iran was importing 43% of its petrol last summer, not almost all of it. They have a lot of room to cut consumption. Petrol is heavily subsidised, ludicrously cheap, & used profligately as a result. There's also smuggling of petrol out of the country on a vast scale. In fact, I wouldn't be at all surprised if some of the apparent consumption is "ghost" consumption, the same petrol going round a few times, collecting subsidies each time.How feasible is a blockade?
Iran has only one refinery, and imports almost all of it's gasoline. We would be hurt if they shut off the crude, but they would be crippled.
The government is currently trying to end imports, in order to reduce the cost to the exchequer, as the retail price is much less than the subsidised price. It's doing this by the ineffective means of rationing, as cutting the subsidy would be politically painful. Blockading their petrol imports might do them a favour. They could use the blockade as an excuse to abolish the subsidy, smuggling would cease, people would try to economise on petrol, the government would save billions, & the country would probably get by on domestic production.
BTW, Iran doesn't have only one refinery. From the US Energy Information Agency (http://www.eia.doe.gov/emeu/cabs/Iran/Oil.html - to pre-empt false accusations of making it up) "Major refineries include: Abadan (400,000-bbl/d capacity); Isfahan (265,000 bbl/d); Bandar Abbas (232,000 bbl/d); Tehran (225,000 bbl/d); Arak (150,000 bbl/d); and Tabriz (112,000 bbl/d)." New refineries are being built & the existing ones expanded.
Iran is able to import petrol overland, & via the Caspian Sea. Unless Russia, Azerbaijan, Kazakhstan & Turkmenistan co-operated, a blockade would raise their costs, but not cut off supplies. There's some scope for importing overland (at still higher cost) from Turkey, & more limited & at higher cost still, from Pakistan.