InterceptRadio.com Forums

So, odd situation at hand. Over the past 2 weeks, I've put on and taken off my new rooftop cargo box. Since I'm an MPG dork, I've kept careful track of my results with and without the box.

City (without)- 21.4 mpg
City (with)- 21.2 mpg

Highway (without)- 28.1
Highway (with)- 28.9

So, with a 16-cubic foot cargo box, at highway speeds, I've increased my gas mileage. WTF?


Note- my mileage would probably be higher if the car didn't have two speeds- "fast" and "off".

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President-in-Exile, Seattle Area Radio Communications And Scanner Traffic Intercept Crew (SARCASTIC)
Don't feel bad if you can't use your STD100/200; there are still people using Digital Frequency Search!
Bunnery definition for the under-fives

A little free advertising :D

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Jim N7UAP - Bellingham, WA / InterceptRadio.com

Certainly no expert but from all the reading I've done on Formula 1 body design... I'll give it a shot.

My guess would be that the airflow is continuing over the roof of the car; however, the container is compressing the airflow slightly. So instead of the airflow aimlessly scooping over the windshield and bleeding off, it's being directed and thus reducing drag. I would guess you're seeing the efficiency boost only at higher speeds.

A F1 engineer described how their carbon-fibre body parts were constructed with .0001" accuracy and implied that it doesn't take much to throw performance off. These engineers are going for either A. speed or B. balance -- of down-force and/or drag reduction -- usually depending on the track design.

I have to agree though, that I wouldn't have expected this either.

Without being too picky, you also have to take in a lot of other variables when measuring MPG. Temperature, altitude, weather conditions can all affect your measurements.

"Dream Girls" is truly the WTF here.

And the 'No Parking' sign you ripped from the fence.

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a) Make damn sure you're comparing apples to apples, the noise in the data due to outside air temperature and wind will hide any effect of the configuration. This is why we spend big bucks on wind tunnels.

b) I see two immediate side effects of the box. It alters the fore/aft balance, which could raise or lower the nose and directly affect drag of the basic vehicle. It also masks the rack hardware (you didn't remove that, did you?). To a first-order approximation, the rack bars themselves have as much drag as the box alone.

I didn't remove the crossbars for box-less testing. I figured a16-foot box would be more of a drag factor than the 2 oval shaped "Whispbars".

_________________
President-in-Exile, Seattle Area Radio Communications And Scanner Traffic Intercept Crew (SARCASTIC)
Don't feel bad if you can't use your STD100/200; there are still people using Digital Frequency Search!
Bunnery definition for the under-fives

I always new that winter temps/fuel blends affected mileage. It is definitely a different animal as there are some other issues going but our 2011 Prius sees a nosedive in mileage during colder spells

City driving summer 48.7mpg
Highway driving summer 51.3mpg

Winter city driving 39.2
Winter highway 43mpg

Hopefully temps are not messing with your numbers.

The last 4 days with the box have all yielded an average of .8mpg higher, including an 80mph+ trip to Olympia last night and a 75mph (averaged) trip to and from Cle Elum.

_________________
President-in-Exile, Seattle Area Radio Communications And Scanner Traffic Intercept Crew (SARCASTIC)
Don't feel bad if you can't use your STD100/200; there are still people using Digital Frequency Search!
Bunnery definition for the under-fives

Well, if you go even faster, you'll get better mileage! :D

"Trooper -- no really, I was just gathering data to better enhance my MPG efficiency..."

I bet if you slap a Google sticker on the front of your cargo container he may let you off with a warning. ;)

Wouldn't it be easier to hoist bodies in to the rear cargo area?

And doesn't that block your NMO mounts?

Interesting that your highway MPG is higher. Also interesting is that you're looking at an ~18% reduction. If Priuschat is anything to believe, a 10% drop is normal. And supposedly this 10% number is on all cars; so those with trucks that are only getting 18 only see a 1.8 MPG drop which is easy to shrug off.


As to Wil's problem, the sample size is too small to draw any conclusions from. I suggest signing up for Fuelly where you can track MPGs and add tags (like "body bag on") to differentiate between fillups for easy comparison.

The crossbars and supports can create a lot of drag, and the box may reduce the airflow that gets to them. The box itself went through some careful design, it's less draggy than the original car based on area. But I wouldn't bet that the box actually improves net mileage - just that it doesn't do much harm. Best way to prove it would be to get to a really stable cruise at constant throttle, then jettison the box and see whether the car accelerates or decelerates.

Of course, the reason highway mileage beats city is that you're holding constant speed. Going 80 implies you weren't in bad traffic...

Except on Badger's Prius he's using the electric motor to aid with acceleration and cruising which is why the EPA rates it higher city than highway.

1 NMO mount, and that's only blocked with the box all the way back. If I shift it forward to where it slightly overhangs the windshield, the antenna mount is exposed and there's still no appreciable loss in fuel economy.

_________________
President-in-Exile, Seattle Area Radio Communications And Scanner Traffic Intercept Crew (SARCASTIC)
Don't feel bad if you can't use your STD100/200; there are still people using Digital Frequency Search!
Bunnery definition for the under-fives

I am not too detailed about tracking it anymore. The first few months I was. I think the EPA said 51/50 or something like that. With the city getting better mileage. While I can easily get 48+mpg city driving in the summer without trying and 52-53 trying hard, I have gotten upwards of 57mpg on quick runs down to Ilwaco with just me in the car.

It is a great mileage boost from my 1994 Land Cruiser... I used to get worried about gas prices.. Now not so much. There are plenty of people out there who will get more upset than I will if gas gets close to $4.50 a gallon again.

Plus I don't keep track of when it is with the family (wife and two kids in car seats)

I like cheese too.

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