[pvcoach]

There's not really just one answer to this one as there are several factors involved here. The frontal area of the trailer and the truck both have contributions, but you also have to consider the peak engine performance of the truck, the environmental conditions (wind direction & speed, temperature, humidity, etc), the friction factors of both the truck and trailer, and a few others. Most of these factors go well beyond this discussion. Since we all have very little influence over most of these conditions, the one thing that we can control the most is vehicle speed. The force of the air against the frontal area of the truck/trailer, multiplied by velocity (speed) we are traveling determines the force of the air against the vehicles. So, the bottom line here is if you want to improve your MPG, slow down. My experience with our set-up is that 60 MPH works best for us (~13 MPG pulling the Monty). (Oh, and yes, I'm a Mechanical Engineer! Sorry...)