Guess the mileage drop on this Prius C with loaded roof rack

[Cd]

So my neighbor has this Prius C, and gave me permission to post to EM.
Year round, she drives with an empty roof rack, and sees " about 42 MPG "
( This is just the uprights and two crosspieces, and not the setup you see here. )
I asked her to glance at the F.E. the next time she drove.
Trip was " around 65 mph on mostly highway. "

The couple normally drive a Subaru suv / station wagon type vehicle, and her boyfriend says he only sees around a 4mpg drop with this same setup on that vehicle.

This is their " Battle Prius " as they call it.

So what do you all think ?

[Piotrsko]

Could be lie-O-meter mileage, my golf reports 50, the gas pump says 43. I thought a 10% minimum reduction, but I don't know what the reported should be.

[oil pan 4]

I got about a 20% range loss putting a 52 inch light bar on my leaf. Good thing I went with the magnetic mount.

[Cd]

I'll let you guys know the answer next week.

[Fat Charlie]

My 98 Suzuki Esteem got around 40. Put an Xmas tree on the roof and go on the interstate? 20 mpg. I voted 31 mpg here.

[Cd]

35 MPG was the result.

[Stubby79]

I win!