I'm around 10 mpg combined on my truck (86 D250 318 auto with 217K) out a single exhaust with a Flowmaster 40. Back in '90 i drove my restified 70 340 Challenger as a daily driver while attempting to find work in the Atlanta area. The engine was a non-original 70 340 rebuilt by Muscle Motors in Lansing, Mi. and when i kept my foot out of it would easily get 20mpg+ combined over the course of 40k miles before i sold her.
