I wrote the following on another thread. . A good percentage applies here... unless you can change total weight, wind resistance & acceleration do not expect more than a 1-3% increase... It's not in the math... .
This is related to a air dam on the top of the cab to deflect air over a trailer but the basics apply here...
my quote
After spending some time with a engineer and having several hundred hours with a chassis dyno I have to put my 2 cents in...
A good mechanical engineer can plot a graph for you based on weight, acceleration, wind resistance and through this formula predict with in 5% of a vehicles mpg... .
I would like to add that my wife gets better MPG than I because she drives slower and accelerates slower... and VW is looking at removing out side rear view mirrors and replacing them with a monitor because of the change in wind resistance.....
I do not believe, after what I've learned that you could achieve more than a 1-3% increase in fuel economy over the current bubble used on modern 5er's...
I own (2) 5er's as well as several work trailers... . work trailers up to 22K lbs... let's cover the 5er's only... one is a mid 90's and weighs 10,500 lbs and has a very simple slanted front, the second is an 06, weighs close to 13,000 lbs and has a nice bubble... . (better design). The 06 tow's better and we get the same fuel economy with the newer trailer. The newer trailer is 14" taller and several feet longer than the old trailer.....
So if you have an older trailer you might maybe see 3-5%... .
Thanks for listening.....