Roof Rack/Cargo Box
 

Just purchased a Kia Niro hybrid. Avid Triathlete and Mountain Bike enthusiast. Plan on taking some longer trips. Considering putting a roof rack with a cargo box from Thule or Yakima for additional and secure storage. Any one have experience with such. I know there will be a decrease in fuel efficiency, but to what extent? Is there a particular brand or model with less of an effect on fuel efficiency? So far I have only found general discussions on the internet. Nothing specific.

Is it possible to put the bike on the roof with a cover to keep wind driven rain out of the headset ect. or have you considered that and ruled it out ?

There are some basic comparisons here:

9 best car roof boxes | The Independent

No info on affect of fuel economy though. I remember as part of our driving test, we learnt about the effect a roof rack/box can have of fuel economy, on average they can use up to 30% more fuel. I'm sure with modern aerodynamic designs things have changed a bit since then!

Getting a hitch to put bike on back. I have this set up on my last car and it allows me to lock it very securely to the rack with a Ubolt and Kryptonite "Forget about it" chain and lock. I more interested in any current data on fuel economy with cargo box on top.

Specifics will be hard to come by because car shapes differ. The increased weight will have a negative affect, but putting things on the roof more greatly increases the drag on the car, and thus lowers efficiency while cruising.

The drag area of a car, or anything for that matter, is the drag coefficient multiplied by the frontal area of the car. Increasing the frontal area is going to have an equal opportunity increase on drag no matter the car. The increase to the drag coefficient of a slippery car, like most hybrids, from a roof rack is going to have greater impact than on something like a truck.

A roof rack and rails will increase the drag and frontal area of a car alone. In terms keeping the impact on fuel economy done, get the shortest rails for the job. A roof rack dam/spoiler will help, but perhaps not enough to justify the cost. It might be worth it for the noise reduction. For the cargo box, I'd go with a long narrow one over the shorter square one. Less frontal area is the main reason, though it is easier to design for low drag coefficient with a longer item.

Have you looked into a combo bike and cargo rack for the hitch? There are several types out there. I would also suggest checking a Niro specific forum. The car has been out for nearly a year or so in Europe. Owners there might provide better insight into what racks have the least impact on fuel economy.