Gas is sold by the liter–about 3.8=1 gallon.
Most places, you never get cheated.
But in Quintana Roo (CANCUN), Mexico, I’ve experienced a couple of attempts. I’ve written to the Quintana Roo State Tourism office, to no avail–perhaps they did hear me, because I bought gas later and there were two guys, one checking the amount, the other making sure I saw the $00.00 on the pump!!
However, two friends had the same trick pulled on them at the first gas station coming into Tulum from Cancun: they got only about a quarter of a tank for $200 pesos (the tank fills with $200 on that little ATOS car), and then were told that they had NO oil and that it cost $150 peso per liter (really $50). Shame on you Quintana Roo. [SEND COMMENTS TO SECTUR and Quintana Roo Secretaria de Turismo]
In many places in Mexico, you can now use VISA cards to charge gas. But not in out of the way places. Best to carry cash. A fill up in an economy car is about $200 pesos.
1. pull in, unlock your gas tank, tell them what you want (you want the low octane, unless you’re driving a luxe car). It never hurts to say, (in English or Spanish): buenos dias/ noches.
2. get out, stand in front of your car, and check the $00.00 on the gas pump. This is where they screw you over. Once they told me, after charging me $200 pesos, that I’d asked for 20 liters.
3. get a receipt (una nota, not una factura, which is a tax document). Take a photo. Go to the office, if you have a problem.
4. I’ve only had this problem in Matehuala, SLP (once 30 years ago) and between Cancun and Akumal, QR. (twice in 2010).
5. the rest of the time, the gas pumpers are great. If they wash your windows, tip them $5 pesos. I often tip the pumper $5 pesos or so–and ask for a receipt. It never hurts to say, (in English or Spanish): ‘gracias.’