Welcome!
I think that 15 could be doable in an El Camino, with lots of hard work that is. The first thing would probably be to change driving habits (in SF it's kinda tough because of all the traffic) then do some mods. Some wheel skirts would be nice to see on that thing, as well as some kind of aerocap that we see on trucks here.
Like others here are saying, V8 addictions are not good for FE at all. I can get 8 MPG in my Civic going WOT the whole time (I wish I was kidding). Be sure to read up on the 100+ hypermiling tips. The best mod that you can do to your car is done to yourself!
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