Discounting the tow rating of a tow vehicle
Why we recommend discounting the tow rating of a tow vehicle
by Carl Lundquist
Let's get back to basics here.
First of all, what is the purpose of discounting tow rating? It is to allow the purchase of a truck/trailer combination that will be a pleasure to tow in the conditions of western North America. Those conditions include altitudes ranging up to 11,000 feet in passes and 6,000-7,000 feet on plateaus, 5-10 mile steep interstate and highway grades, and high speed traffic.
A novice buyer, and most folks are novices in the purchase of RVs, will be faced with a range of trailer prices, floorplans, and weights to mate with a tow vehicle with an even larger range of capabilities and prices. We have recommended for years that the decisions be made so that the trailer GVWR is less than the tow rating of the truck, van, SUV, or car that pulls it. This allows the buyer to eliminate unsuitable combinations in sorting out the various options available, without having to rely on the self-interest of sales personnel.
The altitudes of the west have a significant effect on normally-aspirated internal combustion engines; a loss of 3% of rated HP per 1000 feet of altitude. This is not a matter of carburetor adjustment or computer reprogramming of fuel injection. It is simply the lack of oxygen molecules being sucked into the engine cylinder. Less oxygen, less combustion to provide power.
Turbo- or super-chargers act to pack extra oxygen into the combustion chambers to increase the amount of combustion. A turbo'd diesel or Ford EcoBoost engine, therefore, suffers a loss of only 1% of rated HP per 1000 feet, a trivial loss compared to 3%. An un-turbo'd gas engine loses 21% of its power driving down the main street of Flagstaff AZ. A turbo'd diesel loses only 7%.
20% is just a rule of thumb to help a buyer buying a trailer/truck to tow in the far West. Will it result in overkill? Maybe. In this respect, trailer towing is like hunting grizzly bears; There may or may not be such thing as OVERkill, but there is sure such a thing as UNDERkill.