Originally Posted by Northernmedic
There are many factors that will effect performance and MPGs while towing. Frontage on a fifth wheel tends to exceed that of holiday, cargo, and boat trailers. Frontage will add to your drag co-effiencent and will require higher torque hence RPMs to counter-act. It is extremely easy to acquire extra weight in a fifth wheel once loaded. Clothing, food, propane, water, bedding, etc... add up very quickly. Also tire inflation pressures on trailer tires is crucial, so is the tire inflation pressure on the truck.
Your absolutely correct your points about weight and tire inflation. Your also correct about drag being the biggest concern. The only point I'd make is it isn't the additional frontal area of the 5th wheel that makes the difference. In a trailering setup the front area only accounts for approximately 25% of total drag and much of that is already accounted for by the front of the truck. Where trailering really increases drag is from three areas.
- Side and under body - The trailer is obviously responsible for a high percentage of the under vehicle and side area drag compared to the truck. This is turbulence as well as friction from the greatly increased surface area.
- Rear of the trailer - the blunt end of the trailer is very inefficient compared to the tow vehicle by itself.
- Gap - the disrupted airflow between the pickup and the trailer is added. It's not necessarily that high (maybe 20% of total drag) but it's non-existent if your not towing.
This may be more information than most want but the important thing to know is pulling a trailer puts your Drag coefficient much higher and that makes the biggest difference in your MPG for long hauls because of it's relation to speed (velocity). Yes weight is a factor; yes RPM's are a factor, yes rolling resistance is a factor but at highway speed drag is the biggest factor. It can account for up to 2/3 of the fuel used. As speed increases the drag goes up quickly because drag increases at the square of the velocity. Our trucks have a decent coefficient of drag so towing really blows that to hell.
Assuming you truck is running properly, if you want to save gas with a trailer simply slow down. Everything else is playing with minutia in comparison.