Americans love tacos. We ate 4.5 billion of them last year — that’s 490,000 miles of tacos! But like so many foods, the origins of the taco are unknown. Some historians theorize they date to the 18th century silver mines in Mexico, because the word “taco” referred to the charges the miners would use to excavate ore. These pieces of paper were wrapped around gunpowder and inserted into holes cared into the rock — Not totally unlike a delicious flour tortilla wrapped around perfectly seasoned beef, cheese and tomatoes and inserted into your mouth. Whatever it’s origins, here are places you can celebrate National Taco Day, Oct. 4 on the cheap: Baja Fresh: JoinClub Baja and get a free taco. Chuy’s Tex-Mex: The Austin, Texas-based chain will give away a crispy beef taco with any order for $1. Chuy’s is also offering $1 Mexican beers. Del Taco: Raving Fan club members get two free…