Not sure if this is earth shattering research, but I took a long trip with a few adults and a trunk full of stuff and the MPG meter showed an average of 3 MPG less than last weeks long trip driving solo.
Guessing that there was an extra 400-500 pounds.

 

So it took about 10% more fuel to move about 10% more mass. Is that about it?

 

Not sure, but with gas prices where they are...I'm going on a diet!

 

Based on the Prius Simulator, I calculate the effect of the added weight would be 1.5 mpg. There may have been some other factors also at work such as more A/C, wind, etc.

 

Haroldo,

Don't EVEN mention the mileage penalty hauling around the extra weight if one of the adults was your wife or girlfriend....

 

This is a good point. I know my trunk is full of stuff now and I am going to empty it out now...

This is one of the long list of items that can improve mileage. They all add up!

Bill

 

Would filling up your tank only half up all the time help with weight? What's that like 70 pounds? Probably not worth the hassle of going to the gas station twice as much though.

 

Technically speaking, every pound matters.
NASCAR race cars try to end the race on fumes, fearing that lugging around the excess fuel will sap much needed energy which could go into speed. Airlines have to decide (within FAA regulations) whether to fly with full tanks or not so full tanks.
I doubt it would matter, one way or the other, since the difference between your mileage would barely be perceptible, IMHO.
Better option to keep the trunk uncluttered, since you can drive without 'junk in the trunk', but not without gas in the tank.

 

70 pounds of gas will reduce your mpg at highway speeds by 0.2 mpg. Weight matters, but other factors are far more significant. Like air pressure in your tires, maintaining a constant speed and slowing down using the generator.