I found this under hypermiling tips. I believe it to be incorrect in several ways.
Safety is well-enough represented, but what of utility? What can my vehicle do, and how do I minimize costs without minimizing safety? Utility is a vital part of "economy".
106) Avoid towing
Trailer towing delivers the triple whammy of increased weight, higher aerodynamic drag, and a third (or fourth) set of tires for more rolling resistance.
Carry loads in the vehicle if possible.
If not, minimize towing speeds and adjust your technique to account for the extra momentum the trailer and its load will add.
To reverse the order, point two
Carry loads in the vehicle if possible goes against all advice and experience I have seen in re long-term economy (longer drivetrain life due to decreased engine load).
1,000-lbs loaded onto a trailer is much easier for a vehicle to deal with than the same load placed in the tow vehicle. A pair of draft animals can transport a heckuva lot more with a wagon than some magickly balanced load distributed on their backs. Same with soldiers dividing up a load onto bicycles or a cart versus humping same.
Point one is a bit ridiculous.
Of course one will use more fuel, but, following manufacturer guidelines, it is well within the practical use parameters of the vehicle. Rather like saying,
never carry any passengers. I buy a vehicle to do work, this is obvious. Could be your fragile little eco cars aren't worth much (air conditioned go karts), but most of us need practicality in our single do-it-all vehicle. And that assumes a bit of robustness.
No, the real question is properly matching the cargo trailer (CT) and tow vehicle (TV)
and being certain the hitch rigging is dead-certain.
A well-balanced rig -- TV & CT -- according to actual weight on the axles derived from a scale (
CAT Scale Search)
where the load on the steering axle has not decreased; or, has increased according to formula is the basic rule. Trailer tongue weight (TW) needs to be 10-15% of the trailer weight (again, generally; depends on trailer type); and TW needs to show up on both TV axles AND CT axles when all is said and done.
There is more to it than this -- the devil is in the details -- but this is the basic way of perceiving this problem.
Let's assume an enclosed cargo trailer, tandem-axle. Dry weight of 2,500-lbs with a 2,500-lb load capacity for a CT gross weight rating (GVWR) of 5,100-lbs.
The TV is a larger car/minivan or pickup.
An ideal TW would be (13%) 650-lbs. Now, let's assume that our TV can deal with this load with proper hitch rigging and we have perfected same with trial & error attempts. In other words, the two vehicles weights & capacities are not an impediment to moving the load SAFELY.
What is the mpg problem?
According to a White Paper published by both Cummins and Kenworth -- for the big truck industry -- the single impediment to mpg is
Below 50 mph: rolling resistance
Above 50 mph: aerodynamic resistance
So, my enclosed CT should have, at a minimum, appropriate load-rated tires designed for least rolling resistance on the road; a "highway tread".
And here is what is missed by many:
The axles should be aligned, professionally.
The brakes should not drag or bind.
The suspension should have shock absorbers.
One wants a state-of-the-art brake controller in the TV.
One wants a hitch that prevents -- not just damps -- trailer sway. (All trailers sway, it is only a question of how much).
The trailer, empty, should roll straight. Loaded, same (load must be balanced side-to-side, not just front-rear.
Assuming that the TV has been checked in the same way (drivetrain reliability questions concerning heat management, brakes, etc) then
the only thing left is aerodynamics.
The White Paper[s] go on to explore aero aids. The biggest "problem" is airflow between the TV and CT. A gap
wider than 30-inches is the airflow killer for the hitch rigging,
and,
I would state, the one least likely to be remedied in other than a purpose-built vehicle. (My truck and [bumper pull] travel trailer saw a gap of just over five feet, as an example).
What remains is CT height, and width.
A width the same or less than the TV is ideal. (And easier to deal with in traffic for sightlines and general maneuvers).
Height is the real mpg killer
once we have adjusted for all other conditions as found.
So, if my 5,000-lb trailer is the same width or less as the TV, and is no taller
my fuel economy is not so adversely affected. Length, weight, etc, are not so nearly important. Give me a longer, but lower, trailer.
Second is an overall aerodynamic shape. RV'ers have a hard time understanding this one, but I know this crowd won't.
No square corners, and enclosed undercarriage are great.
But pale next to height.
I'd rather have the trailer above: optimized rolling resistance and best aero compromises present when I'm contemplating a load for a trailer.
We all have need, on occasion, where a trailer is the best choice. We can all imagine a scenario -- weather, earthquake, fire, war, etc -- where time and our choices are limited AND our vehicle is the only way to do it.
Economy is best with optimizing details in that situation. Mainly, a road speed around 45-50 mph. And spare fuel aboard.
In the steady-state world, 58-60 mph on the Interstate (roughly, 10 under).
For "peacetime" then some study, and some investment into the best tow vehicle and matching trailer -- cargo, construction, travel, etc -- pays dividends.
The third point, [above] is to adjust driving. Of course. One will need more power at many points: acceleration, braking, etc. It isn't hard with practice
when one understands that he is the slowest, worst-handling vehicle on the road.
The trailer I used as an example is the largest enclosed trailer offered by U_Haul at under 6' tall and 12' long. I loaded it and my truck, and, with an estimated gross weight of between 12 and 13,000-lbs I saw a solid 19 mpg to move over 3,000-lbs of goods on a 300 mile trip. I could have loaded both vehicles to a higher level had I another sort of goods and likely returned the same or similar miles.
My 300-mile fuel cost was (at $2.6/gl) $41. Or, 13.5 cpm (cents-per-mile).
The industry of moving things (trucking) looks at that: all expenses divided into miles to understand cost.
Previous to this I was averaging 15 mpg with a 7,500# 34' travel trailer at a higher road speed. I could likely have broken 16 mpg had I dropped some rpms/speed. This is consistent with some others I know of with similar trucks and trailers; or, simply: a diesel pickup and a lighter weight aero trailer; where the scale tickets showed a gross of nearly 16,000-lbs with a 9' high trailer.
Mpg, solo (7,360#; at the higher speed) is 22+ (versus 15 towing the TT; 16,000#).
Mpg, solo (at the lower speed) is 24+ (versus 19 towing the CT; 13,000#).
Diesel is the magic when it comes to towing. High compression just flattens out the hills. A manual transmission means all miles are within the "sweet spot" for the engine: accelerating, cruising, etc.
A gasoline powered 3/4T truck of the same make & vintage would average, (per others experience) around 7-9 mpg towing, and 15-17 mpg solo. Even if higher, you can see the "diesel benefit" (exclusive of initial cost [offset by greater capacity and longer life; i.e., buy a used truck]).
And so forth.
If you understand your cost-per-mile (all costs, including depreciation, finance [foregone investment], taxes, insurance, fuel, repairs, maintenance, tires, etc) then the "cost" of towing is one where vehicle life is maximized, safety is never compromised and driver condition is comfortably alert over a long day.