Roof crap effect on fuel economy

[Jonathan Moxon]

I've noticed about a 1 mpg decrease when I have my ski rack on in the winter time.

Sponsored

 

[STW]

My bud runs a RTT on his lifted JLUR.

His mpg drop is 4 mpg less by adding the RTT. I get better mileage pulling my camper than he does with his RTT. He will average about 12 mpg with the RTT and I average about 13 mpg pulling this camper. This is comparison is done as we travel thousands of miles around the US together.

Both JLURs are running 37s, lifted, and re-geared to 5.13.


His RTT adds a bunch of noise to the Jeep while driving and a bunch of buffering sounds. I have rode in his Jeep for many many miles. There is zero chance I would add a RTT.

Interesting. I use an iKamper Mini roof top tent and maybe its more streamlined hardtop shape makes it less of a drag on mpg? I put it on a low profile Maximus-3/RhinoRack, and don't hear any wind noise or buffeting when it's up there. 23zero 180 awning mounted across the back so it's a bit out of the wind back there.

I don't have as many miles measuring the effect it as you and your friend, but I did a 2K mile trip last June with the iKamper up there and it seemed to have a relatively small effect on mpg. On the Canadian Hwy 93 portions of the trip at 60 mph or below, I was getting a solid 23mpg (2.0T 2door Rubicon, on stock 33s then). Once back on US freeways at higher speeds, mpg dropped to 20 mpg.

 

[Bug Splats]

While I don't ever bother to measure my fuel economy in any scientifically accurate way, based on the MPG readout on my dashboard it made no difference when I added my DeeZee roof rack bars. FWIW.

Many variables to take into account, but I’m leaning towards agreeing with C.Sco here at this point.

I’ve got about 1500 miles on the odometer, but since adding a DeeZee rack before the 1000 mile mark, my average MPG on the dash has mostly been climbing, currently a little over 18 MPG driving ~70-75MPH flat highway with my son and I.

 

[Ahre]

Lateral Resistance/Drag increases logarithmically with speed. You got’a pay to play/go fast... Put it in the right lane, slow down, smell the roses and enjoy a good night's sleep in a plush RTT.

 

[Ahre]

My bud runs a RTT on his lifted JLUR.

His mpg drop is 4 mpg less by adding the RTT. I get better mileage pulling my camper than he does with his RTT. He will average about 12 mpg with the RTT and I average about 13 mpg pulling this camper. This is comparison is done as we travel thousands of miles around the US together

Both JLURs are running 37s, lifted, and re-geared to 5.13.


His RTT adds a bunch of noise to the Jeep while driving and a bunch of buffering sounds. I have rode in his Jeep for many many miles. There is zero chance I would add a RTT.

Ok, who is hiding behind the trailer...

 

[JINO]

My jeep came with a roofrack, so I don't know what MPG was prior.
But when I bought it, it was about 26 or 27 mpg, with the roofrack.
When I added 37" tires, went down to 25 mpg.
Then got wheels and 3.5" lift and front driveshaft, down to a little less than 24 mpg.

Just got back from a trip to the desert with containers on top, with recovery boards and trasheroo. Was getting 23.4 mpg going and coming, about 100 miles each way.

I noticed the most drag from the lift.

 

[wibornz]

Ok, who is hiding behind the trailer...

Probably some random chick wanting to stow away in the camper, or my wife. Not really sure. I find random people checking out our set up often in parking lots. If I was able to zoom in enough I could verify if it was or was not my wife by her toes.

 

[Anonymous]

Probably some random chick wanting to stow away in the camper, or my wife. Not really sure. I find random people checking out our set up often in parking lots. If I was able to zoom in enough I could verify if it was or was not my wife by her toes.

@wibornz I was able to zoom in and confirmed it’s your wife. See!!!

 

[TectonicDiscoveries]

I took a noticeable 2-3 mpg hit after installing a full GOBI rack on the Jeep, which I should of expected, but not as much as it was. This is unloaded too. Most of the time its been loaded with kayaks and such its fairly short trips so haven't paid attention to the range then.

 

[Sidewalk]

The Jeep is as aerodynamic as a brick, adding crap to the exterior isn't going to make a lot of difference. A modern minivan is pretty sleek, doing ANYTHING to disturb the air is going to do a lot of damage to MPG.

 

[Shibadog]

Ok. Given that anything which will increase drag will reduce fuel economy we are safe in saying there was a “hit” of some sort. The car mentioned is a LOT more aero than our Jeeps. That hit sounds high, BUT without knowing just how badly it screwed up the drag on the car we can only guess. I know bars and a roof rack knocked the already poor economy of my 98 TJ down enough that I removed it asap.

 

[Maverick909]

Lateral Resistance/Drag increases logarithmically with speed. You got’a pay to play/go fast... Put it in the right lane, slow down, smell the roses and enjoy a good night's sleep in a plush RTT.

This. my best mileage is 65-70mpg. anything past it goes down pretty fast

 

[wibornz]

@wibornz I was able to zoom in and confirmed it’s your wife. See!!!

Lol, not her. She would never be able to get her feet into a decent set of hiking shoes or hiking boots.

I think you got that pic from my Only Fans foot page. Thanks for the $189.99 monthly subscription payment. Just kidding the subscription is .42 cents a year.

 

[whitechocolate]

So.. you're saying we shouldn't add a rooftop spoiler for more downforce?

F is for Family

 

[_olllllllo_]

how aero was the Kia in the first place? If it's quite efficient aerodynamically, then adding bars could make a larger % difference. The photo suggests a fairly boxy SUV so maybe this doesn't explain much.

But in general, our Jeeps are so inefficient aerodynamically that adding something like crossbars may not make as much % difference as it would for a more aero car shape.

Three big factors make a car aero or not:
1) frontal area. larger is less aero.
2) coefficient of friction (fr). This has to do with the airflow across the shape, both a) how many little things obstruct smooth airflow (rain gutters, windshield hinges, lol), and also b) how the overall shape allows smooth airflow, such as whether the backend shape creates a turbulent low pressure zone, etc., and sedan roofs are carefully shaped nowadays to reduce a common turbulence.
3) aspect ratio--longer shapes are better aero shapes. Maybe this should be part of #2 I don't know, but it's a big factor.

Our Jeeps are bad at all three, haha, and 2doors are worse since they're shorter than 4doors but with the same frontal area. Lifts create more frontal area, #1, as well as making more exposed friction obstacles below the Jeep, making #2 worse.

So how much a roof rack will change a Jeep's overall aero effect is may be less than for a vehicle actually designed to be aero.

You also need to calculate for the air passing under the vehicle. All EVs have very smooth undercarriages with minimal areas for air disruption outside of the suspension and wheels, which have a large affect. Luckily our Jeeps have all kinds of extra stuff underneath to affect the airstream. Look at so many of the EVs of today and they look fairly boring and similar. A 1950's vehicle has tons more character and sex appeal than the jellybeans they are whipping out today in the name of low drag.

Sponsored