Tempest unreal fuel charge

We rented a Renault Sandero Stepway 900cc for 10 days, from 7-17 July 2018. We needed to fill up the vehicle after day 8 of using it. We have by then travelled already 790km on the one fuel tank. We were so impressed with this little vehicle's fuel consumption since we got roughly 17 km's to a liter. It was 44.6 liters and it cost R692.20 (R15.52 per liter) to fill up the 50 liter fuel tank. We were so impressed with the vehicle we looked at the specifications on Renaults website, and the website confirmed the combined cycle (urban cycle plus extra urban cycle) to be 5.2 liter per hundred km and we got almost exactly the same results at 5.56 liter to hundred km's. When we dropped of the vehicle at the airport we only travelled an extra 48km's since we filled it up. Thus we travelled in total 838km's. When we received the bill Tempest charged us R285.30 for petrol, according to them 18.49 liters. We queried it since the vehicle's fuel was almost full and the petrol to fill up the vehicle could not have been R285.30. It should have been roughly between R40-R50 to fill it up. The branch administrator emailed me and said that I must sent my fuel slip and they will calculate my fuel consumption. Which I did but I still have not heard anything back from her. She asked one of her co workers to phone me and argue with me that this vehicle took the R692.20 (that I put in the vehicle and got proof for it) plus the R285.30 (that they charged, but can't give me proof of the garage slip) to drive only 838km's. I kept on telling the lady that it is impossible to use R692.20 + R285.30= R977.50 to drive 838km's. That would mean that, that little vehicle will get only 1.16km's per liter!!!! That is impossible! But Tempest refuses to give me answers and proof. I asked for a garage slip, that they can not give me. I also asked for a tracking report (since all car hire vehicles should have tracking devices) to show that I only travelled that 48km's since I filled up the vehicle. And CCTV footage at the garage that the vehicle was actually there to be filled up after we have returned it. Because the figures does not add up. If they claimed that they filled up the vehicle with 18.49 liters of fuel after we returned it, it would mean that we travelled roughly 323km's from Ballito to King Shaka Airport. Which we did not, and a tracking report will proof that! Now no one is getting back to me or answering any questions that I had. I want my money back since I see this as *****. I used to work for rental company and knows how it works, and it is very easy and straight forward if you do it honestly. But they are just robbing people and thinking they can get a way with it! Every 200 odd rand charged extra from every rental agreement is a lot of money at the end of the month. ****************!