Mass (weight is great for static stuff) in rotational bits really isn't a great raw measurement. You can consider that, as a component of the car, you'll be using energy to change its linear direction just like any given fixed part of the car (headlight, battery, spare tire, door, Little Tree (R) ), but that's the limit that any two flywheels, pulleys, wheels/tires, etc. can be compared when using a scale.
You've got to account for the moment of inertia. How far is the average mass from the center of its rotation? Take a given mass on a piece of string and spin it around, and again on a longer piece of string. The further from the axis, the more energy it takes to change the speed it spins.
Two flywheels that read 13 lbs on a scale will
not necessarily perform identically in rotational acceleration.
This is also the reason that lightweight pulleys are bling; a crank pulley's moment of inertia is dwarfed by those of the flywheel, the wheels and other bits that make those 2.6 lbs. at a 1.5" radius almost inconsequential and certainly not something that you would notice in straight line acceleration.
Wheels are a different story; they're plenty larger than pulleys in diameter, making changes in their raw mass typically much more influential. Moving the moment of inertia further from the center with a larger diameter wheel fights speed changes substantially, and gyroscopic properties make more effort in steering.
So, unless you're down to the ragged edge or being very cruel to the hardware, g'won and choose your pulley by its appeal to the eye. Don't spend too much time going on about how it took .3 sec off your quarter mile, though?
