Mileage loss with a Thule roof box

Discussion in 'Gen 4 Prius Accessories and Modifications' started by Erin Lang Echo, Jan 24, 2023 at 9:55 AM.

  1. Erin Lang Echo

    Erin Lang Echo Junior Member

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    Has anyone had one? How much gas mileage do you lose?
     
  2. Isaac Zachary

    Isaac Zachary Active Member

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    No, but yes you would lose fuel mileage. Here are my thoughts:

    If you primarily use it driving around slowly (e.g. around a neighborhood) the loss wouldn't be at all noticeable. The faster you go down the highway the higher the loss.

    Putting a Thule box on backwards (pointy end towards the rear and not towards the front) will probably be more aerodynamic and cause less drag than putting it on the way most people do.

    A cargo carrier that hangs off the rear of the vehicle will cause less aerodynamic drag than something on the roof.

    When driving on a highway, most of your energy loss in a gasoline powered vehicle is engine efficiency losses. The second greatest loss is aerodynamic drag. Two factors that improve or worsen drag are the crossectional area and the drag coefficient. Adding anything to the top will hurt both of these. (Hence, why a rear luggage rack doesn't affect fuel mileage as much as it doesn't affect crossectional area).

    I've known a few Gen 2 and Gen 3drivers who had ski racks on the top of their Prius and complained about getting higher 30's lower 40's MPG. But they also drove around like maniacs.

    But if you need more luggage space you need more luggage space, plain and simple.

    It's also safer for you not to fill your rear seat with stuff, and also to tie down anything you put in the rear hatch. (I knew a lady who died in a 25 mile an hour single car accident because she had stuff in the rear cargo area that popped up and whacked her in the head when she slid diagonally off an icy road). Also filling the rear of the vehicle to the ceiling cuts off your rearview mirror view.

    Your other option would be to buy a bigger vehicle, but that costs more money.
     
    #2 Isaac Zachary, Jan 25, 2023 at 10:21 AM
    Last edited: Jan 25, 2023 at 4:05 PM
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  3. PaulDM

    PaulDM Active Member

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    5mpg with a thule weekender
     
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  4. CR94

    CR94 Senior Member

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    I. Z. is correct that the increase in fuel consumption caused by cargo on the roof will be highly speed-dependent.
     
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