Mileage loss with a Thule roof box

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.

 

5mpg with a thule weekender

 

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

 

I did a cross country road trip in 2021 with a large Thule roof box and I average high 30s and low 40s MPG. I tracked it on Felly and probably drove at around 70-75 MPH most of the time.

 

Mileage losses with aerodynamics are non-linear to the speeds driven. Drive faster with something like that and lose a lot more fuel economy.

 

Right! It's related to the square of the speed.

Plugging in some numbers here https://ecomodder.com/forum/tool-aero-rolling-resistance.php I couldn't find gen 4, but gen 3 I put 3050 lbs, 0.10 Crr, .25 Cd, 23.4 sq. feet of frontal area and .28 engine efficiency, and it all calculates to 47.9 mpg at 65 mph, which may not be accurate, but let's just go with this for now.

If we add more weight, say 700lbs, we get 44.6 mph at the same speed. Mind you that in a real world the engine efficiency would slightly increase with increased load so fuel mileage would not drop that much.

But now let's mess up the aerodynamics and keep those 700lbs of people and cargo. I'll just guess 2 more square feet: now we're at 42.3 mpg at the same speed. But let's also hurt the coefficient of drag by .2, and now we're at 40.2 mpg at 65 mph. Of course, again, under greater load the engine efficiency will increase slightly so in a real world, if we added this much weight, area and hurt the coefficient of drag by that much, we'd probably be over 42 or 43 mpg still.

But let's go faster. that 40.2 mpg at 65 mpg drops to 36.4 mpg at 70mph. At 75 we're now at 33 mpg. And at 80 we're doing 30 mpg.

Let's try a 4 square foot box that hurts Cd by .4:

65 mph = 36.4 mpg / 70 mph = 32.8 mpg / 75 mph = 29.6 mpg / 80 mph = 26.8 mpg.

Of course again, the numbers wouldn't be that bad due to the engine's efficiency increasing as a greater load is applied to it. Still, it would be possible to get worse than 30 mpg depending on how big the box is, how much it upsets the aerodynamic shape of the vehicle, and how much weight we're adding, as well as how fast we are going.

Now if you put that on a cargo carier behind the vehicle:

No increase in frontal area, slight decrease in Cd (I'll guess .1). And we calculate (drum roll)...

65 mph = 44.6 mpg / 70 mph = 40.5 mpg / 75 mph = 36.9 mpg / 80 mph = 33.7 mpg.

 

Nice work. Love it. The OP should be sending you a beer or a coffee.

 

Jeremy Clarkson said it best while driving a Bugatti at its top speed. It has very low drag CD. Clarkson said something to the effect that if you want to increase speed from 250ish mph to an extra 10 mph? You need an extra 100hp - because at that speed - driving through air is tantamount to pushing a car through fruitcake.
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I know That this is a Gen 4 question...I have a Gen 2. Recently took a trip back to Colorado with a Thule Adventurer Box and snowboard rack full plus the family and trunkspace full. Had a 40 mph headwind which was the worst I've had making that drive in the past 16 years.

Box was hanging off the front windshield but I might try flipping it around. Normally I get 48mpg. in the winter. about 38 mpg with the box when we take trips and I drive about 80mph. So I figure 8-10 mpg. Worse case. A Gen 4 is more fuel efficient than the Gen 2 so I can't image yours would be worse that that. Hope this helps.

 

Best way to take a smaller hit on a high efficiency vehicle (or any vehicle for that matter) is to use a trailer hitch carrier. Even these will take a hit on efficiency - as it disturbs the car's slipstream effect... but it's not as bad. Even extra cargo weight will affect efficiency as there is greater friction between asphalt and tire rubber. This is our hitch carrier setup - which adds over 500 lb including the weight of the carrier.


Less expensive than Thule - too

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For sure. Next trip out west we plan on using one of these.

 

Technically, if you can build some sort of aerodynamic tail on the rear of your Prius (or any car for that matter) you may actually get even better fuel mileage with your added storage in that tail:

https://www.aerocivic.com/

 

The last time I took my Prius out with my cargo box, I think I got 38-40 on the highway - but that was with 4 people, a bunch of stuff and there may have been some times where I drove over the speed limit.

But, to be able to take 2 adults and 2 kids in interior comfort, with all of their stuff and still get close to 40 mpg is pretty cool.