When I'm hauling a 2nd car on a dolly with my truck going similar speeds, sometimes I see an increase in MPG. I think it's more of the boat tail effect than the weight. I've hauled scrap and all kind of misc stuff in my truck, weighted down it coasts farther, adjusting the driving style for that and I normally don't see much of an MPG effect unless I have unpredictable stops like driving through the city with stop lights.
There's a saying I heard that sprung weight reduction is like 10x that of unsprung, but I'm pretty sure the context of the saying is for racing and probably relates to handling + acceleration + braking. Spinning weight has a larger effect than non spinning effectively when you're trying to change speed.
There's "batteries" designed as a spinning mass in a vacuum on magnet bearings to have as little friction as possible. Google says those flywheel batteries can be up to 90% efficient. Clearly the longer the time between charge and discharge (self discharge rate) will hurt that figure.
Either case, with more weight, just pumping up the tires to max sidewall helps offset the rolling resistance added by the weight. There's clearly an advantage with going lighter but for steady state speeds weight doesn't have much of an effect.
Most extreme example I can think of is a semi truck, fully loaded it looks like they loose around 30-35% fuel econ due to weight, but they are running around with and extra 24 tons using the figured posted. My Prius is just under 1.5 tons to give context. I'd say the areo effect is much much larger lol.
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Depends on the truck engine/trans/rear gearing and the type of trailer. Our Volvo that pulls a flatbed gets 12 mpg bobtail(no trailer), 10mpg with trailer(deadhead) and around 6.5–7 loaded(48,000lbs payload grossing around 79,000lbs). Our Kenworth that pulls the same trailer type and weight gets around 7.5–8 loaded. And around the same deadhead.
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https://www.quora.com/What-s-the-mil...l-trailer-load