i wanna know more bout slant sixes

somthin ive heard of for helpin with gas milage is a hydrogen generator im not exactly sure how it works but i got a buddy buildin a small one its supposed to get 10% better gas milage supposedly there is a more complicated version that gets you 75% better but hes starting small first i may do that to a slant... how much gas milage does a stock 318 or 340 or 360 get if i could get one of those to get 25 mpgs.... power wouldnt be a issue