I called Visa Credit Card before my trip to Mexico. They were helpful; the first CSR transferred me to Benefits CSR who answered some questions but not all; then that CSR transferred me to another CSR who was well versed on the car insurance benefits that Visa offered. She also told me the internet link for me to print the benefits policy. I was covered by the credit card. When I got to Hertz San Diego Airport the Hertz agent convinced me to take the Mexican insurance based on problems we may encounter if we get pulled over or into an accident. At $25/day I thought it was worth it. You should see the paperwork they give you.
Note: Insurance is not paid per 24-hour period but for the entire day (e.g. you rent from Monday at noon until Wednesday at noon - that's 3 days of Mexican insurance - Mon-Tues-Wed; for American Insurance that's only two days).