There are many complaints about the lack of numbers. I looked up the study [0] and they indicate that they pull their numbers from the U.S. Energy Information Administration and EPA. The methodology is summarized [1] and explained in detail [2] if anyone wants to go through it.

[0] https://www.andersoneconomicgroup.com/many-gas-powered-cars-...

[1] https://www.andersoneconomicgroup.com/second-edition-real-wo...

[2] https://www.andersoneconomicgroup.com/wp-content/uploads/202...