The cheapest way to fly from one city to another in Spain is to use free stopovers if they are included in your transatlantic fare. Sufficient free stopovers may permit you to fly without additional charge within Spain. Today three others add competition and have lowered prices that were already reasonable. The quality of a Spanish road and whether it is multi-laned are indicated by its letter designation. These are usually one lane in each direction and pass through towns and villages where speeds must be cut in half or more.
Gas is a major expense in Europe, costing three times what it does in the Unites States.
Gas prices are quoted by the liter, which is slightly more than a quart. An international driving permit is required for foreigners driving in Spain. Branches of the American Automobile Club can supply one. Incidentally, AAA has reciprocal arrangements with automobile clubs in Spain that provide similar services for AAA members from the United States. Note that Spanish rentals will bill at 12 percent more than the price charged, because of the IVA tax.