The testedvehicle has 252,500 to 263,500 miles during the test period. The vehicle does not have extra equipment added to improve towing performance.
The 3 test configurations were:
(1) No trailer; loaded with estimated 500 lb cargo
(2) 4x8 enclosed U-Haul trailer. The basic empty weight of the trailer is about 850 lbs, and it was loaded with estimated 900 lbs cargo. The total weight 1,750 lbs
(3) 5x9 open utility U-Haul trailer. The basic empty weight of the trailer is about 1,200 lbs, and it was loaded with estimated 700 lbs cargo. The total weight 1,900 lbs
There are 3 types of resistance forces acting on a moving vehicle, i.e. aerodynamic drag, rolling drag, and hill/grading drag. For a given vehicle traveling at the same speed, flat terrain with no wind or little wind conditions, the only variable will be the rolling resistance. From many of Caterpillar RV/Truck design handbooks; the rolling resistance is typically proportion to the total vehicle weight. One (1) HP can overcome. about 650 lbs of rolling drag. So, the difference in these test configuration would only had contributed approximately 3 HP difference. Therefore -the majority of HP requirement differences will be attributed by aerodynamic drag, and grading drag.
In this first test data set:at 70 mph, flat and no wind condition.
(1) No trailer configuration - 21 mpg
(2) With enclosed trailer - 17 mpg
(3) with open trailer - 12.5 mpg
At 70 mph - 21 mpg = 3.33 gallon / hr;
17 mpg = 4.11 gallon /hr and
12.5 mpg = 5.6 gallon /hr
At 70 mph, 21 mpg, the engine generates 57 HP, assuming 1 gallon of gas/hour produces 17 HP.
For the enclosed 4x8 trailer, it saw an increase of 0.78 gallon/hr fuel consumption.It translates into a 13 HP of trailer drag. For the 5x9 open trailer, the fuel consumption increased to about 2.27 gallon /hour. That translates into a 39 HP of open trailer drag.
The rolling resistance of 4,700 lb vehicle and a 1,900 lb trailer is about 6,600 lbs. The rolling drag will be around 9 to 10 HP. The calculated aerodynamic drag for the vehicle was about 47 to 50 HP.
The significance of this set of data reveals that the open 5x9 trailer generates 80% more drag. Even enclosed 4x8 trailer with long tow bar (big gap between trailer and tow vehicle), this configuration only generates 26% more drag.
From the second test data set, traveling:at 70 mph
(1) With enclosed trailer - 15 mpg at flat and 5 (0 to 10) mph wind condition. 4.66 gallon/hr
(2) With enclosed trailer - 17 mpg at flat and no wind condition. 4.11 gallon /hr
The difference of 0.55 gallon/hr fuel burn due to head wind will require additional 9 to 10 HP. Theoretically, the drag force is proportional to the 3rd power. Traveling at 70 mph with 5 mph wind vs traveling at 70 mph no wind would translate into 23% more drag. The theoretical HP requirement calls for 47-50 HP * 123% = 60 HP. The test result and theoretical calculation of fuel efficiency were the same.
Conclusion:
The rolling resistance, or vehicle weight isn't a big issue, the aerodynamics is. Having heavier vehicle due to added aerodynamic treatment is a better solution than focusing on the weight only. If a vehicle is constantly traveling in 3% graded plus area, then the priority should be on reducing the vehicle weight.
