Talking about fuel, I check mpg with each fill up and in years when bio become standard in fuels, I noticed about 2 mpg drop.
Right now my wife drives gasoline MB as diesel wagons never made it to US, but I am driving diesel Ford.
Lately got some jet fuel to burn and even jet fuel has lower BTU, my mpg went up.
Hard to say by how much and mpg on dually always is pretty inconsistent, but my instant ScanGauge from 14 went on couple of occasions to 17 mpg.