MPG Increace

I am another one who got 30mph with a 78 aspen rt with the 318, 883od /2.96 gear, on the highway with the usual hot rodding stuff , ld4b elderbrock intake . a 360 2barrel cam, duel point dist , duel exhaust. this car went from 15mpg new to 30mph after changes on the highway. and would put a hurting on many ford/chevy and one very upset roadrunner with a 383 (he didn't believe me it was a 318 ) that's why his milage numbers don't seem to add up to me. so did you change speedo gears in the trans may be its not reading right?