Horsepower with mileage options

some check their mileage when they take a trip and then tell everyone what their mileage is.But that is only 2 or 3 % of the time for most of us.An od seems to be the logical choice,and for highway driving with a cam that is most efficient at the od cruise rpm this is probably the case.However it may be terrible for some engine combos for lower speed applications..I have a friend who installed an od in his 3.91 geared 408 truck and his mileage went down!