Had this discussion with an automotive engineer pal. His thoughts: The weight in the loaded trailer causes it to travel straighter and more smoothly, meaning the tow vehicle has an easier job perambulating over the road, hence, higher mpg. The empty trailer will have a tendency to "bounce" over the road a little (and there was a spewing of engineering terms and formulae here that I absolutely could not follow!). This means the tow vehicle is meeting resistance in moving the trailer forward, hence, lower mpg.
Don't know how true this might be, but this is a smart fellow who regularly hauls live and inanimate cargo behind his own truck, and it may make a degree of sense. This is also assuming no additional factors were in play during one trip versus the other, and that the mileage calculator in the vehicle is accurate and not given to fluctuation.
Last edited by coloredhorse; Nov. 19, 2011 at 10:14 AM.
It takes fuel to move weight. The more weight you have to move the more fuel you'll use. That's why a loaded Freightliner gets 5 mpg and VW Rabbit Diesel 40 mpg.
My first explanation might be "electronic anomaly." The DIC type indicators are notorious for their lack of consistent accuracy. If you did this with a manual calculation you still have a chance of error if you're just relying on one tank full. Keep a log over a couple of dozen trips and see what the numbers show.
Terrain, wind, and load distribution can alter mpg numbers. Speed and "driver input" are even bigger factors. So is traffic congestion (more speed up and slow down means more braking and acceleration, meaning reduced mpg).
So maybe what you saw was real and maybe it wasn't. You need to do multiple trips to know for sure.