Although a highly biased example consider my brother in law. He commutes from Manchester to Leeds every day. He used to fill up three times per week spending £60 each time. He now fills up once a week.
The worst example was a courier who used to fill up twice a week for £80 a time. He used to get 350 miles a tank. He then bought a £40 bottle of XFT and got 400 miles a tank saving £80 a month. He cancelled because he was only saving £40 per month!!!!