Most has already been said. I just wanted to add a few observations:

- Iran is very safe, I was never concerned about the amount of cash we were carrying. If you do need some extra peace of mind, a number of Iranian banks offer "tourist credit cards". Just step into a bank, open an account, deposit your cash and you receive a card that works everywhere in Iran

- If you did not bring enough money it is still possible to get money (Dollars or Rial) with a credit card or a Paypal account. Look for the larger money exchange offices and ask around. They will not be able to help you officially, but they will tell you where to go (I got a nice glossy flyer with an address and a map). You'll end up in some anonymous apartment building with a flat that is filled with computers that have a permanent VPN connection with the outside world. Make your Visa/Paypal payment online and walk out with your cash. Commissions are high though, up to 10%.

- prices for fuel are the same for foreigners as for Iranians, at least in all the places we fueled up (around 10 eurocents per litre if I recall correctly). Most fuel stations have permanent attendants so no need to worry about "fuel cards".

- as mentioned above, all prices and price discussions are in Toman, not Rial. If it says 50000 that's Toman and means 500000 Rial.

as to how much money you will need, that's really hard to say. It will vary depending on your needs. (Hotel) Accomodation is rather expensive, especially in the more touristy places like Tehran, Esfahan, Shiraz, Yazd, ... But most cities also have a nice park where camping is allowed (and free), often with toilets and shower facilities (Tabriz has a really -nice one, with a security guard). Iranians love to camp, so wild camping is a pretty good option as well. On top of that, especially in less touristy areas we got so many invitations to stay at people's homes we stayed in a hotel only once and spent the rest of our time camping or staying with the most hospitable people you can imagine.

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Belgium to Nordkapp via Iran | Discover Overland | Destination World