The " fuel economy " is based on how heavy your throttle hand is. The - gas - difference between the 4.3 and 5.0 is minuscule. The 5.0 has more torque which means its going to get up on plane with less gas than the 4.3. Has more cylinders which means less wear and tear on the engine if your cruising at the same speed as a 4.3. The 5.0 will be better for water sports bc of the torque and will be able to keep a steadier speed for sports like wakeboarding. The biggest difference between a carb engine and EFI is in starting up. Now most EFI engines are newer so there is usually a few more refinements in the heads that may help your fuel economy. For the average 50 - 100 hour a year boaters that may translate to about 50.00. My guess is if you get 2.0 mpg at cruise with the 5.0 and the 4.3 is newer in design you might get 2.1 with that. 3,000 rpm is usually the sweet spot for optimum fuel burn. So you may be cruising at 27 mph with the 4.3 and 28-29 with the 5.0 at the same rpm. I have a couple 4 barrel 5.0s in one of my boats with 1300 hours on them, 150 lbs of compression across the board on the cylinders , pushing a 9,000 lb hull ,smooth as silk at idle and as easy as it gets maintaining them.. IMHO great engine as is the 350 which I have owned as well