| This month, I would like to focus on some of the issues surrounding fuel efficiency when towing fifth wheels and travel trailers. The discussion started when I received the following message from one of the RV Lifestyle readers,
John Easlick.
Here is what John had on his mind – I am sure that many readers are wondering the same thing…
Dear Andy:
I subscribe to RV Lifestyle Magazine, and I am in the process of purchasing a truck to pull my 2006 Rockwood 8283 model with GVWR listed at 8081 lbs., however I believe I will never be towing more than 7,500 lbs. The fifth wheel is 30 ft long, and the pin weight is listed at 1081 lbs. It has a rounded front cap and is a mid-profile unit, so I think it should tow easier than some of the taller models.
I have done quite a bit of research on what size of truck to purchase as I don't want too much engine for the size of my trailer. In Andy Thomson's recent article, labeled towing with the GM Mid-size SUV’s, the person has towed a 31’ Airstream with a Buick Enclave. The article doesn't state the weight of the 31' Airstream, however it must be at least 7,000 lbs. The tow rating of the Enclave is 4,500 lbs. That is a significant difference; however the driver was quite satisfied with the towing ability.
To tow a 7,000 lb trailer, most truck specifications state that one would require at least a 5.3 liter engine as shown in their brochures. Am I missing something? I really don't want to purchase more engine than I really need. I might be towing my trailer 6 to 12 times per year, and I don't care if I can race up a steep grade. I have been considering a GMC truck since I have a few thousand dollars in credit card points however I really just want to purchase the "best value truck" for my needs.
Your opinion of what to purchase would be greatly appreciated.
John Eastlick.
Hi John:
Thank you very much for your letter – I am sure that many readers have similar questions, so I will try to address the issues for everyone’s benefit.
Though weight is a factor in towing, once the weight is moving it is aerodynamic drag that really determines how easily a combination will move down the road.
Though mid-profile fifth wheels with more streamlined designs do tow easier than earlier conventional shaped fifth wheels, they still have considerably more aerodynamic drag than an Airstream, and the Enclave is fairly aerodynamic for an SUV and considerably more aerodynamic than a pickup. When combined, the pickup and the fifth wheel trailer would have the equivalent of 60-70 horsepower of aerodynamic drag at 55 miles per hour, the Enclave and Airstream travel trailer would be closer to 40 hp of drag.
No one currently builds a full sized pickup with an advanced V/6 engine but even if they did another problem would present itself and that is the power curve of the engines. Though the 3.6 litre has as much peak horsepower as many V/8 truck engines, it occurs at a considerably higher RPM Range. So while the 3.6 litre has plenty of power to tow the Airstream along at around 2300 RPM it would not have enough power to overcome the resistance of a truck and fifth wheel without dropping to a lower gear, likely in the 3500 RPM range. Shifting down is fine for climbing hills and acceleration but you don’t want to have to drive all day at high RPM’s just to overcome air drag.
Since 2001, we have had a two wheel drive half-ton GM pickup with a 4.8 Litre V/8, 3.42:1 axle ratio and 225/75R x 16” tires. I have never had a pickup that gave the fuel mileage this one does. I would hate to tell you what we have put this poor truck through, but so far, we have 180,000 kilometers on it with no serious problems. From a power perspective nobody ever guesses that it is just the 4.8 litre when they tow with it. This truck runs 27 MPG on the highway and 12-13 MPG when towing a mid-profile fifth wheel such as yours. The towing mileage is not much better than a truck with a larger engine but the solo mileage sure is. If you don’t absolutely have to have four wheel drive stay with a two wheel drive model as it will get better mileage especially in cold weather. A limited slip rear axle is a good compromise as it improves traction in snow but it does not cost any fuel mileage.
Today, you cannot order this exact truck from GM - but you can come close. The 4.8 Litre engine is still available and you can now get it with a 3.73:1 axle ratio. The standard tire size is a 245/70R x 17” which results in a little more rolling resistance than the 225/75R x 16” tires that we use on our older truck. It is important you do not get the larger 265 tires that are available. It will perform a little better with the 3.73 axle and it may get slightly better mileage towing. As well, the new truck is quieter, “tighter” and more comfortable.
The new truck is taller and seems to create a larger profile to push through the air, so solo mileage may suffer a little compared to our 2001. It seems that truck buyers keep buying trucks that are taller and boxier; I don’t understand this, as a taller truck has a higher center of gravity and has to push through more air. I can see it for serious off road use, but most people don’t do serious off roading and certainly there is no reason for it in a two wheel drive. One of our customers tows with a restored 1970 Chevy half ton pickup. It is so much lower than the new trucks that it almost feels like you are driving a car - you don’t need running boards to get in and out of it and loading the box is much easier. The old truck basically does the same job as the new trucks do without the excessive height. It seems to me that if a truck was designed with modern drive-train technology, a lower profile with more attention to aerodynamics and sensible tire sizes, and less attention to looking tough it would be considerably more fuel efficient and much better handling. If you had built a truck like that last year it would not have been a big seller, but things may be changing now.
There is some new technology heading to half-ton trucks over the next few years which will improve the fuel economy. There will be some smaller diesels, and more transmission gears. Ford is working on a program they call Echo Boost which will see a half-ton get a very heavy duty turbo-charged 24 valve 3.5 litre V/6. This will allow V/8 caliber performance when towing and V/6 fuel mileage when running solo.
You often see people driving down the highway with an empty pick up with the tailgate open to conserve fuel. I actually used to do this once in a while myself. A couple of years ago, Transport Canada actually tested the theory in the wind tunnel and surprise - there is less drag with the tailgate closed! It seems that with the tailgate closed an air pressure is created in the box that helps fill the void behind the cab. A Tonneau cover does help mileage when not towing. Though the mileage difference is too small to determine for sure I think a tonneau cover from the cab to just in front of the hitch does seem to help when towing. The theory is that it dumps the air over the sides of the box which is a cleaner air flow than going down into the box.
If you can borrow someone’s truck to take your trailer to a scale it might be a good idea to confirm the pin weight before you purchase a half-ton. Though total weights are pretty accurate these days, brochure pin weights can occasionally be a long way off.
One other thing to watch for is the box size. For years we had two box sizes. Long boxes which were 8’ long and short boxes that were 6’6” long. Now there are often three box sizes: the Long box is still 8’ long, but the 6’6” box is now called a standard box and a short box is 5’6” long. The above measurements are inside opening of the box - not the overall length. For towing a fifth wheel the 5’6” box does not allow enough clearance for maneuvering, and while it can be made to work with an automatic sliding hitch, it is still not ideal. I find a 6’6” box with a sliding hitch works well. The combination is more maneuverable than a long box and the ride is better due to less chassis flex.
Thanks for writing and I hope you can put those GM points to use soon, the open road awaits!
Andy |
