I went through there a few years ago and had awful problems getting fuel for the first few days. We finally managed to get a fuel card off the mayor ( at least I think he was the mayor ) of Yasd. We entered into the country from Pakistan and never new about fuel cards untill we tried to get fuel. The locals are very helpful but they don't even get enough fuel for themselves so it's hard or them to part with some of there rations.
As far as I know you can get fuel cards at the borders and if you can make sure that you do, It will make everything much easier.
Enjoy Iran, it's a fantastic place with the friendliest people.