You are not way off. There are many considerations to take into account from a scientific standpoint, and it seems too many just look at their now inaccurate dash display and draw the conclusion that MPGs have suffered.... I've been trying to figure the math out for my 2016 Transit 350 XLT...Anyone know if I am way off on these or not?
More than a basic understanding of physics and math must be applied in order to accurately analyze the difference, which is beyond what most of us have the tools to determine.
Never mind that when you choose to drive a big brick around that has different utility than a Prius, a few MPGs should not be something to overly worry about.