It is difficult to get a regional map. The Guia Roji is highly recommended. Plan ahead and either make reservations or know where you might want to stop to eat or spend the night. Watch the level of gas in your tank, and be sure to keep it at least half full. While there are plenty of Pemex gas stations along the major highways, in rural areas stations are less common.
Gasoline is sold by the liter - just a little less than a quarter of a gallon.
However, Mexico is phasing out its leaded gasoline. If you need it a lead additive, be sure to carry it along as some gas stations do not keep this in stock. The best precaution is to avoid driving at night, since most highways are not illuminated and you stand a good chance of driving into a pothole or a black cow that has strayed from the field. One terrific service needs to mentioned here. It doesn't hurt to leave a tip, but that's completely voluntary.