Curious about people's experience with a cargo box on top of their prius. How many mpg on average did you lose with a cargo box? I'm considering a thule cargo box and will be going on an extended road trip.
Not a bad idea, but I imagine with the added back-end weight and drag, you're losing around the same MPG. Also, extra wear on the back tires / potential for loss of stability.
I bought a Sportrack rack and Yakima RocketBox Pro 12. I took about a 4-6 mpg hit, but that was also driving 70+ mph for long stretches.