I'm planning a long bicycle tour from Hungary to Asia across many countries this summer. I was at the Iranian embassy at Budapest, Hungary where the ambassador told me that I must apply for a transit visa if I want to cross Iran by bike, entering from Armenia and leaving to Turkmenistan. And I must have the 'to" country's visa already (=Turkmen). The problem is that I only get the Turkmen transit visa if I have both the "from" and the "to" countries - so I must already have an Iranian visa there (+Afghan or Uzbek, the "to").
There is no need to get a transit visa but it is better to apply for a prearranged one before starting your trip. So fill in the visa form on Iran visa application , it takes about 3-7 working days to get your reference number and two working days to collect your visa at the embassy by direct referral or one working week by posting the docs to the embassy. You can pay the visa fee in cash in Iran.
Check the cheapest Iran tours .