360 engine/ headers?

Even though I have read many posts where headers improve fuel mileage I can't say I've seen much improvement from the vehicles I installed them on. Usually stays about the same to just slightly better. Mainly just power improves. I did have one that lost mileage as you say yours did. Turns out it must have been jetted right on the edge of lean with the stock manifolds so when I put the headers on it was really lean. So lean that the throttle had to be pressed farther down to get it to go. I jetted the carb 2 sizes richer and the fuel mileage went up to where it originally was and the power increased dramatically.