Guntherman

I recently noticed a dive in fuel economy. Some theories:

• Winter gas
• Tires wearing down so now they're smaller diameter, screwing with shift points, programming etc.
• Increased headwinds because of changing seasons
• Aging vehicle

Then I remembered my mpg dropped right around when I installed my toolbox. It's a Delta single lid crossover, high profile, nothing fancy. I watched the 2009 F-150 aerodynamics video where the engineer talks about how the truck is supposed to direct airflow toward the tailgate which is designed to efficiently transition the airstream to turbulent flow, minimizing the coefficient of drag. I noticed immediately after installing the toolbox that morning dew and rain accumulated and moved around on my back window, when it had not previously. My prevailing hypothesis became that the inefficient aerodynamics of the toolbox interfered with the efficient aerodynamic design of the truck.


As an engineer, I immediately set about testing my hypothesis by trying to find a 3D model of an F-150 that I could import into Autodesk's Project Falcon aerodynamic modeling software. I became frustrated at the absence of free 3D F-150 models on the internet and also became irritated at not understanding the nuances of the software.


That's when my wife, the psychologist and voice of reason, said, "why don't you just take the box off the truck and see if your mpg improves."


Long story short: I was getting 16.7 before. Now I'm back up to 18.6. I tried to keep all conditions the same to not skew the results of this experiment. I'm fairly confident in the results after 800 miles or so.


Hopefully this helps someone trying to solve their mysterious decline in fuel economy.

Truck owner

That is incredible that a tool box located in bed would have a 2 mpg difference. I cant believe the extra weight would cause that much decline, but you just proved it does. Wow,, these trucks are ultra sensitive.

mechanicboy

Picture of said toolbox? How much weight did you carry?

BE25FX4

must be a heavy toolbox!

Terminex

How much does the toolbox weigh, and what is the weight of everything in it?

I do not remember exactly, but it believe the estimate was 1mpg for every 100lbs of weight added.

Not sure how accurate that is, but I would guess the weight has more of an impact than the air flow.

I think both together are the factors.

JWM1986

Your theory makes sense but I would've never guessed that a toolbox could have that much effect on MPG. Thanks for the info tho!

Guntherman

It's mainly an aerodynamics issue, not so much weight. I still actually have the box in my bed, it's just not sitting on the rails. Here's the video I mentioned earlier:
I'd be interested in finding a solution to make the toolbox more efficient aerodynamically. Maybe gaspods or those turbulators they sell for truckers to jump the airflow around the truck-trailer gap. Or maybe I'll just get a box that sits under the rails.

Adobe2X

Well, the tires getting smaller would actually result in an increase (although false) in your MPG, as each rotation would be shorter than before. That would cause your odometer to register more miles than actually traveled.....just sayin'

jcloving

I remember seeing an episode of Mythbusters and they said that the open bed with a tailgate up is the best case for aerodynamics. A large amount of air rides over the cab and down, hits the tailgate, then goes up the back glass. The tumbling air then allows more air to ride over the cab and the air in the bed.

Their conclusion was that mileage suffers if you hurt this rolling pillow of air. Driving with the tailgate down would hurt it.

It sounds like your toolbox might do the same thing, ie. disrupting the air in the bed.

lewis26

iTrader: (1)

hmm, this is interesting. I have a low pro UWS toolbox, and I wonder if it is affecting it as well...