**Visa information can change at short notice**
Hopefully it hasn't escaped your attention that Iran does not enjoy harmonious diplomatic relations with most of the Western world at the current time. If you're travelling through Iran you need to be aware that the visa regulations for this country change frequently.
Before you think about an Iranian Visa you’ll need to get your hands on an "Authorisation Code" - this is issued by the Iranian Ministry of Immigration in Tehran and takes up-to 6 weeks to process - just to keep things simple this does not guarantee that you'll be granted a visa, it simply gives you the ability to apply for one.
Iran does not demand that you make your visa applications from your country of domicile, but most Iranian embassies do require you to make your application in person in order for them to collect biometric data from you. You'll also need to think about a carnet for your car.
The Visa Machine have loads of useful information on their website specifically for the Mongol Rally. Here is their section on Iranian visas.