A lot depends on where you buy your fuel. Every Monday on the president's daily two-hour rant of which I watch everyone, there is a presentation from PROFECO (Mexico's BBB) that lists the stations with the highest and the lowest prices. This week, Costco Tabasco came in with a price of 17.96 per liter (Magna) and the highest was almost 23 pesos per liter (Magna).
In many cases, to remain competitive, franchise owners are absorbing the cost.
For those not familiar with PROFECO, apart from consumer complaints, they also list all products for sale in Mexico with the highest and lowest prices in the section of their website called, "Quien Es Quien". Very useful.