i have done the valve job on my engine to 8 and 18 thousandths. i have installed a bhaf on the stock air tube and still no change in mpg. it doesnt matter if i goose it or im driving it like a grandpa, or pulling my 23 foot horse trailer and running empty it gets between 10 and 12 mpg. the only thing i havent done is have my timing advanced. is it safe to advance timing with 276 thousand miles and without head studs?