Should Your Pickup Tow Less at Altitude?

We do a lot of strange things as auto writers (including driving some pretty nice pickup trucks), but one thing I occasionally like to do is look through and read owner's manuals, especially the parts on towing and hauling.

While recently testing a 2015 Ford F-150, I looked through its manual and found this on page 251: "Note: For high altitude operation, reduce the gross combined weight by 2% per 1000 ft. (305 m) starting at the 1000 ft. (305 m) elevation point."

Interestingly, we could not remember reading or hearing about anything like this from any of the other manufacturers before. Clearly, this means that the truck's carrying capacity (found in the gross vehicle weight rating) and maximum towing capacity (found in the gross combined weight rating) would be negatively impacted at higher elevations in significant ways. In fact, this made us wonder why only Ford would offer such advice. Was it responding to some kind of pending litigation or was this just good customer safety awareness?

Defining Terms

In the name of clarity, we need to review a couple of terms:

• Gross vehicle weight rating is the maximum weight in pounds your pickup can carry — which includes passengers, cargo, a full fuel tank and lubes, the vehicle itself and any related tongue weight when towing.
• Gross combined weight rating is the maximum weight in pounds your pickup can carry and tow — which includes your pickup at max GVWR (including tongue weight) plus the actual weight of what's being pulled behind the pickup.

Interestingly, as we discovered, this note in the manual about GVWR and GCWR numbers is not new. We looked at 2013 Ford F-150 and 2014 Ford F-150 owner's manuals and found the same note. We found it in the 2015 Ford F-250-550 owner's manual as well.

Anyone who drives at higher elevations knows that the higher you go, the less powerful your engine feels, especially naturally aspirated gasoline engines. This is something the recreational vehicle industry always has been pretty good about communicating to its customers, as they tend to explore remote campsites in the mountains. We found a good explanation in Ford's 2013 RV and Trailer Towing Guide: "Gasoline engines lose power by 3-4% per 1,000 ft. elevation. To maintain performance, reduce GVWs and GCWs by 2% percent per 1,000 ft. elevation."

What About Turbocharged Engines?

To be fair, most people understand the common sense idea that at higher altitudes, the air is less dense so a vehicle (or person for that matter) is likely to have more trouble breathing as freely as it did at sea level. But what about Ford's twin-turbo EcoBoost? Can't that engine just spin the turbos faster and pull in more air?

Ford Truck Communications Manager Mike Levine said that while yes, "turbocharged EcoBoost engines have the capability to deliver a similar boost level at higher altitudes when the atmospheric pressure is decreasing," this same drop in atmospheric pressure is typically the reason why both power and torque decrease in naturally aspirated engines. While maintaining the same boost level at lower altitudes, Levine noted, other limits can be encountered, like turbo speeds at higher rpms that could also result in a power reduction (at higher elevations), but still have a smaller amount or no reduction of peak torque.

Because EcoBoost engines can lose some performance, Levine said, a reduction in the GCWR is recommended if customers want to maintain the same performance level they had when driving the same truck and trailer at sea level. We would also note that, although not as significantly impacted, the bigger, torque-biased diesel engines encounter the same type of performance challenges.

Why Does This Matter?

The obvious reason is safety. Every pickup sold in the U.S. is required to post maximum ratings and capacities, normally found on the driver's side doorjamb, so drivers know what they can safely carry or tow. Items like vehicle structure, chassis and suspension design, engine type, axle ratios and other mechanical variables are taken into careful consideration when engineers create these vehicles. Overloading a pickup could cause significant safety concerns since the brakes, axles and frame may be jeopardized if the operator is unaware of the safety limits.

Calculating the normal safety limits of your pickup is normally straightforward; you add the weight of your passengers and the weight of your cargo (typically loaded in the bed) to the weight of your pickup (assuming you know the truck's actual weight — don't simply rely on what the manufacturer or salesman tells you). You want this combined number to be below the stated GVWR (found on the doorjamb) for your specific vehicle. If you're towing, remember to add the tongue weight to the GVWR calculation.

When figuring out what you can safely tow, take the GCWR of your truck (a number not always posted on the doorjamb), subtract your truck's weight, the cargo and passengers you're carrying, and the remainder will be the amount of weight you can safely tow.

That all seems like some straightforward math; however, finding out your exact elevation or looking at what elevation gains you may encounter that could lower your GCWR (and quite probably your GVWR as well) 2 percent per 1,000 feet is not common knowledge. Plus, while 2 percent may not sound like a lot, it can have a substantial impact.

A Quick Example

The maximum GVWR of a 2015 Ford F-150 regular cab with a 5.0-liter V-8 engine is 7,850 pounds. The maximum payload is 3,300 pounds for a two-wheel-drive truck with the Heavy Duty Payload Package and 18-inch wheels. That means our F-150 should weigh 4,550 pounds: GVWR minus payload equals truck weight. Most other F-150 setups without the Heavy Duty Payload Package can carry between 1,580 and 2,330 pounds.

At 11,000 feet, like when driving through the Eisenhower Tunnel just west of Denver, our factory-rated GVWR could drop, if using Ford's recommendations, by as much as 20 percent. This means a truck with a payload of 2,330 pounds would need to drop 466 pounds. A truck with the Heavy Duty Payload Package would need to reduce its GVWR by close to 700 pounds.

Maximum towing capacity at that elevation would, of course, also drop as well. Using our same example of the Eisenhower Tunnel and the truck's maximum tow rating of 12,200 pounds — a two-wheel-drive F-150 equipped with a 3.5-liter EcoBoost V-6 and a 3.55:1 rear axle — the towing capacity could drop almost 2,500 pounds when going through the tunnel.

Other Truck Manufacturers

Looking at other manufacturers' owners' manuals, we couldn't find a single reference to reducing hauling or towing capacities at various elevations, so we contacted each pickup maker to find out why.

Chevrolet Truck Manager Tom Wilkinson told us, "Engines do lose some power with altitude, but we have sufficient reserve to handle the payloads and trailer weights at which we rate the trucks."

Nick Cappa of Ram communications said, "Ram is the only pickup manufacturer to use SAE J2807 towing criteria for all three full-size pickup truck segments, and we do not reduce the towing or payload capabilities of our pickups as elevation increases."

Nissan states in its owner's manual that engine performance will suffer at higher altitudes, but says nothing about towing or hauling: "An engine will lose about 4% of its performance for every 1,000 feet above sea level that you travel. If you will be towing in high altitudes, it is a good idea to allow more time than usual due to the engine's reduced performance."

Steve Parrett, manager of Nissan's southeast and south central region communications, said the performance loss applies "specifically for naturally aspirated" engines and Nissan does "not place any restrictions on GCWR as a result of elevation change."

We couldn't find any such note in the 2016 Toyota Tundra owner's manual, and when we contacted Toyota we were told the pickup's towing capacity is not affected by altitude.

No doubt we'll hear more about this issue from Ford and the other manufacturers. In the name of full disclosure, we did contact SAE International several times but have yet to hear back.

Cars.com images by Evan Sears