I installed a bed cap on my Mega Cab and have seen a major decrease in MPG. I thought it would make the truck a bit sleeker, but now Im totally confused. I make a 400 mile trip about 1-2 times per month, and my milage has went from 19-20 down to 15.5 as of yesterday. I had the cap on before the winter fuel hit. Anybody else have any idea's? Thanks

 

tonneau cover or shell? I put a 100 lb hard tonneau on and my mileage went up about .5 mpg...

 

this is a full size shell

 

too much added weight maybe?

btw, welcome to DF..

 

I put a shell on my CTD and the mileage went up about 1 mile per gallon. Mine was Straight line with the cab top and sides.... Is your shell higher &/or wider than the cab? If so than I would say that the xtra wind resistance may be the cause, but I can not understand that big of a decrease! You have pictures. Below is a pic of my shell on truck...

 

Does it raise up higher than the rest of the truck?

The new ultra low sulfur diesel fuel?

Like steve said ... added weight? ( although I honestly don't think it should make THAT much of a difference )

Are you hand calculating your mileage, or using the overhead?

 

I saw this on the MithBusters one night. I could not believe it either. The results they had were for best milage. They tested with tailgate down, up, off, cover , and one of those tailgate nets. The best milage was net, tailgate up, cover, down, and off from best to worse. I was in shock but they did the test with the same truck, same raod, and 2 diferant speeds. They even had a flowmeter taped in to the fuel line!!! Check it out on the discovery chanel. This was on a Dodge dakota I think.

 

The cap fits exactly with the cab lines. I am not hand calculating, just watching the gas gauge. First trip on the brand new truck used just over 1/2 tank, and has steadily used more and more. Yesterday when I got home I was barely at 1/4, no winds to speak of. Driving about 70-75mph. Truck now has 6000 miles on it. Cap only weighs a couple hundred pounds. Had it in the shop last week getting the high idle installed, and had them check for any codes, they said it was ok. I have been running power service white bottle, and I changed the fuel filter at about 5000, just to make sure.

 

Well its just my oppinion but 4.5 mpg less per gal is that some thing else is goin on that is a big drop id take filter and replace and see but cap wil not decrease milage that much.

 

It's called wind drag. I also noticed it when I had a shell on my 01 Dakota with a 4.7L gas engine. The space between the back of the cab and the front of the shell is where it first starts to drag you down. Also you will notice more of the side wind with it on. With the shell on you become a bigger area for the wind to affect the over all drag. The very first time I experienced it and then learned about wind drag was when I drove a flat bed semi truck for a living.