To quote the gas-saving tips at fueleconomy.gov:
Note the “up to”. In city driving, with lots of acceleration, extra mass is going to harm economy a fair amount. During highway driving, extra mass only adds a little to rolling resistance, so there will be little if any effect on fuel economy.
In real-world terms, an extra 100-150 lbs is only going to be noticeable under carefully controlled conditions. Variations in driving patterns will swamp out an “up to 2%” decrease. An extra 1000 lbs , on the other hand, will be very noticeable. I’ve got a little hatchback, and on a few occasions I’ve carried three (ordinary-sized) passengers, or a trunk completely filled with various stuff. When I do so, the acceleration is pretty pokey, and according to the instrument panel MPG display I lose maybe 5-10% of the fuel economy.