I found this quite easy, although the AI behaviour in roundabouts is indeed really bad. You can conserve fuel by switching off the engine
When you start, and left the third light (crossing the wide road) just accelerate to 60kmph and switch off the engine. The simulation is not realistic in this regard at all, so you can coast with that momentum until you reach the highway. Switch on the engine after you turned on it, accelerate to 90-110 and switch it off again. You can do this trick multiple times, but these two places are the best to do it and this will leave you with plenty of fuel remaining to finish the mission with 3 stars. Since there's no time limit either, you can just take it easy. Oh, and park on the right as soon as you see the park message, the GPS has it totally wrong, you don't have to go as far as it shows, so you don't have to take the last roundabout at all.