Howdy,
I just towed my 2230 lb. travel trailer from Kingwood, TX to my summer home near Manitowoc, WI. I kept very accurate fuel consumption records and recorded the mpg after every fill up.
Before towing without the tow bubble, I averaged 15mpg with very sparing use of the a/c in the TV on a trip from Manitowoc, to Yellowstone, to Kingwood, TX and back to Manitowoc. I would estimate the overall length of the trip taken the second week in Sept. 2010 to be about 4500 miles through varied terrain. My return trip from Manitowoc to Kingwood, TX produced the same 15mpg average.
The latest trip with the tow bubble installed and with the a/c on full time set for 77F in ambients up to to 97F, I averaged any where from 15 to 17.3 mpg. Fuel mileage was very dependent on the quality of gasoline.
One major chain with the gasoline cracked by a co-pilot or probably blessed by Pontious Pilate rated 87 octane produced only 15mpg. Racetrack, and Phillips 66, rated 87 octane produced the 17 mpg and the 17.3 mpg tanks. I also blame ethanol for the low mileage simply because of the lower heat value.
Now granted, I tried to duplicate the same terrain as the previous trip by traveling from Kingwood, through Dallas, TX, Joplin, Jefferson City, and Hannibal, MO, then Rockford, Ill, Madison, WI and finally Manitowoc. The Ozarks had some challenging grades: but not those of the Yellowstone area. Most of time the wind was at my back or sides and not a vector wind with which the bubble isn't much help as I discovered on a trip to the Texas Hill Country this spring.
So I can honestly say that the tow bubble increased the efficiency by at least two mpg. I estimate that a/c fuel comsumption is about 1.5 to 2mpg
and as high as 2.5 mpg in city traffic. If I would add that to the 2mpg, the total may be as high as 3.5mpg to 4mpg. Based on the average price per gallon of $3.60 the 15mpg would be $0.24 per mile and the 17 mpg would be about $0.21 or a difference of about $0.03 per mile. The cost of the tow bubble was about $150 that would require 5000 miles to amortize.
My only comment is that if you are designing and building a teardrop or any travel trailer, be sure to consider aerodynamics because in three years gasoline will be at $5.00 per gallon.
Here in Wisconsin, 89 octane gasoline is sold at $3.84 per gallon vs. 87 octane that is sold for $3.79 per gallon. I would think that for the nickel more, this would be the way to go. 89 octane is straight gasoline without the ethanol. Actually enthanol costs more to produce than straight gasoline and would not be competitive with out the subsidy of the federal government.
The next step is turbulators around the perimeter of the flat, squared off rear trailer wall.
Take care,
Maury