The use of geotextiles to postpone reflective cracks in asphalt overlay is a popular practice, so researchers are eager to calculate its structural value. This research study has focused on this issue for geotextiles used in the roads of Iran. Twelve sections from the Tehran-Qom road were tested; each examined before and after construction of the overlay. The tests were of the Falling Weight Deflectometer type, and at least twelve tests were conducted each time. The data from five sections (four for developing the model and one for evaluating the output) allowed a new mathematical model to be developed. For the seven remaining sections, some foreign and Iranian geotextiles were used as interlayers. The mean structural value for all of the geotextiles was equivalent to that of a 2.92 cm-thick Hot Mix Asphalt overlay, while that for only the Iranian sections was equivalent to 2.28 cm. Economic evaluations, based on construction costs, showed that in 2011 the use of geotextiles was economical in Iran, because fuel and bitumen subsidies had been eliminated and different geotextile brands had been brought to market in the country.