620 rear end
I am buying a 1978 620 and it is geared kind if low. What kind of change can i make to improve the gear ratio and power so I can get maximum fuel mileage?
you will have to change the gears in the rearend to a higher gear.
The best thing for you to do to enable different gear ratios is to put a Ford 9" in the truck. Problem is 78 620's only came with 2 ratios that I know of , a 411 and a 423. I am not exactly sure I am correct at a 423, but it is something close to that.
78 620's had 4.37.5 ratios in most and a few 4.11's. I put a 1980 720 rear end in my 1978. It was out of a regular cab with a 5spd manual trans. It is a 3.90 ratio.