I'm glad you asked that. My fiberglass cap, along with a wind deflector (wing) set on the back of it, has increased my fuel mileage noticeably, about 10%. Now, since I have never run my truck without the cap, I can't say what effect it has on mileage by itself. However, the cap helps increase mileage when I put the wing on because it allows the wing to be set high and relatively close to the trailer, thus maximizing its effect on mileage. I have tested the combination over 10,000 miles, and it does work.
I have towed with a truck with shell and without and I promise you there is no increase in mpg. The weight of the shell will absorb any slight and I mean slight increase in milage. If you want a shell get one, but not for mpg reasons. Airing up your tires and watching the go pedal will do far more to increase milage. But folks will still come on here and claim several mpg gain with a cap. PLACEBO.
NCH
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"But folks will still come on here and claim several mpg gain with a cap. PLACEBO"
I don't see where anyone has claimed several mpg gained with a cap but here are some real world numbers. Going over 1,200 miles to south Texas with just trailer 10.2 mpg coming back to Illinois after installing a cap and pulling same trailer, 11.0 mpg. Not much but it certainly did not go down and I appreciate the huge storage gain. On the other hand, without the trailer mileage dropped a similar amoung going from 14.0 to 13.0 mpg with the cap.
I was getting 8.6 mpg towing a 32 foot trailer. I put a Leer high rise shell on (250lbs) and I was recently getting 9.2 mpg towing the same rig. I think having the high rise makes a difference, as it tends to send a lot of air over the top of the trailer. On my F550, the weight really doesn't make much difference.
The cap won't increase you're mileage, but it gives you room to store stuff. See this study. On a personal note my Excursion, which is basically a F-250 with a cap on it got the same mileage as my friends F-250 when he pulled a TT similiar to the one I had.
John (USN Ret) and Debbie
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I've done all kinds of driving and towing with and without a cap pulling U-hauls, heavily loaded utility trailer, and my present 26 foot TT. If there is a difference in MPG with the cap, it has been too small to be noticed. Like others, the greatest benefit to me is the increased storage capacity.
weight matters weather you are driving a ford pinto or a f650... it will only be more noticeable in the pinto...
while changing air flow may help some of the time it will hurt others, unless you only use the TV when the effects are positive you will most likely have no gain...
most people have no idea what their real fuel mileage is and overstate (or understate it) it when talking about it... knowing requires careful documentation over many miles and gallons of fuel... only checking at certain times is no indication of real fuel mileage, but makes some feel better (or maybe worse) about it...
checking on a two way trip discounts to many variables to be reliable...
there is a lot of the PLACEBO effect in play with this topic...
BTY my V10 got 8.1 going south and 8.4 going north this winter... just the opposite of what I expected... and a whopping 9.2 in Florida...
interstate driving at 65 MPH I never made it quite to 15 MPG without the brick in tow...
* This post was
edited 07/07/08 10:57am by JJBIRISH *
Love my mass produced, entry level, built by Lazy American Workers, Hornet
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