Thule Motion XT XL cargo box - impact on mpg?

I've been somehow averaging 20.9 mpg in my 2021 ORP on freeway driving if I hover around 63mph. Concerned that if I get a Thule box I'll see that drop 10-15% per some articles I've read. Is anyone with these boxes seeing this kind of drop?

 

I average 19.2 before and after my roof platform with two rifle cases on it. Made no difference.

20.9 is really good though so I'm sure you might have some small drop, but it probably won't be a lot. The way you drive makes the biggest impact but it sounds like you have that down.

 

It depends if the $700 box is empty and just for looks.
But if it's loaded for bear, that's a different story.

 

I have that box, it really doesn't make too bad of a hit on gas mileage. I ran down to OBX with mine and left it up all week we were there. Mountain driving on the way down and home. It also had 99% of everything we brought in it (4 people). I will say with the stock rack you can't get the bars far enough forward to mount it and have clearance to open the lift gate.