Fill up the tank with “regular” gas.
In Japan there are three types of gasoline available, but generally you should choose レギュラー which is Regular Unleaded Gas. Most gas stations have all their signs written in Japanese, so be careful not to use the wrong type. If you accidentally fill the tank with 軽油 (Keiyu, diesel), the car will not start, and the rental company will charge you an indemnity for the damage. By the way, ハイオク (Haioku) is Premium Unleaded Gas. Since there are usually no English signs, it is good to remember the Japanese characters for these three words.

Full service is convenient but a little pricey.
There are two types of gas stations, full service (フルサービス) and self service (セルフ). Just like other countries, at self service stations you have to fill the tank by yourself, and in turn the price is a little cheaper. It is not very difficult, but if you aren’t sure how to do it you can always ask the staff for guidance. At a full service station, you get the royal treatment. Not only do the staff fill up your gas tank,they will take any garbage you may have in your car,wipe your windshield, and sometimes even open the hood up and make sure everything is OK.

Check the colour of the nozzle!
Each type of gas is marked by a different coloured nozzle. The regular gas nozzle is red. Remember to always fill up the gas tank with the red nozzle! As a side note, yellow is premium unleaded gas, and green is diesel. These colours are the same at any gas station in Japan.

Gas is about 160 yen per litre.
As of Sep. 2014, the price of gasoline in Japan is about 160 yen per litre. In a compact car, you can usually drive about 20 kilometers (12 miles) on 1 litre of gas, so at a rate of 160 yen per litre, you could drive 300 kilometers (186 miles) for roughly 2,400 yen. Make sure you fill the tank up before returning the car to the rental company.

Gas stations are often closed at night and on weekends.
In the countryside, gas stations are often closed at night and on the weekend. If you are driving outside of the city, it is a good idea to fill up whenever you can and never let the tank run too low.