From my trip notes on January 23 '05, driving my SVT Cobra through Mexico. This exact scenario probably would not happen to a motorcyclist, but the fact that there are some crims pumping gas should keep you alert:

Pemex gas station rip-offs are pretty rare nowadays, but they can still happen. At the station on Rt 180 just north of Poza Rica, I pulled in and asked the attendant to fill it up with premium ($2.50 gallon for 93 octane). After he started pumping, I got back into the car to do some paperwork. A few seconds later the attendant called out to me. I looked up and saw 100 pesos on the meter ($9 at 11.15 pesos per dollar) and he asked me if that was all that I wanted. I told him no, fill it up. Then instead of continuing to pump, he reset the pump to zero and then filled the tank. I paid him the 100 pesos plus the final reading. Since I calculate and record my gas mileage for every tank (even doing the liters to gallons conversion), when I did the calculations I knew I had been ripped off and the car could not have taken the 59 liters I was charged for.

I confronted the attendant with my calculations and after some protesting on his part, I was surprised when a fellow attendant handed me 50 pesos, basically an admission that I was scammed. I probably still got ripped for a couple of bucks, since I'm not sure how much if any of the initial 100 pesos of gas made it into my tank.