when carbs are running on their main metering circuit they are at their highest effeciency, it takes a certain amount of horsepower to propel the vehicle along at a certain speed -thus it requires a certain amount of fuel to create that horsepower, so there should be no big change in gas milage if the carb was jetted correctly. But drivability in and about town where there is much transitioning between the idle, enrichment, and main metering circuits should be much smoother than with the more primitive carb. These old trucks usually had 3.90 rear axles and even with 235's could lug around corners with no stumble problems. To get some better mpg a taller rear ratio could now be used and let the efi and big inches really shine in drivability and "no stumbleness". Stout stuff!
