Tow/Haul Mode in ICE Vehicles with Hyper-Aerodynamic Trailers

[turbopilot]

I just completed an 800 mile round trip towing a LightShip with an ICE fueled HEV truck between Arizona and Utah. The trip was a mix of long flat areas of driving mixed with significant mountain grades in both states at high density altitudes and high winds. I made the entire trip without the use of TrekDrive. The efficient use of Tow/Haul Mode with ICE tow vehicles changes with a hyper aerodynamic, moderate weight tow load and changing wind speeds.

At highway speeds, aerodynamic drag is the single largest consumer of a tow vehicle's fuel and demand on the engine and transmission. Because of the LightShip's compressed profile, it slashes passive wind resistance by roughly two-thirds compared to a traditional travel trailer, the steady-state pulling force required from an ICE tow vehicle on flat ground is drastically reduced. However, the tow vehicle is now entirely responsible for moving its full physical mass (an 8,400-lb GVWR) up hills and away from stoplights.

This combination of factors now calls for a re-evaluation of the use of tow/haul mode in an ICE tow vehicle. Previous to the LightShip, I towed a 7,400 lb high profile, Airstream with the same F-150 PowerBoost. Tow/Haul mode was simple with that setup: turn it on and leave it on. But with a hyper aerodynamic trailer a significant amount of efficiency is lost by the constant use of tow/haul mode. Because of the significant reduction in aerodynamic drag by the trailer you don't need tow/haul mode on all the time.

Here are the various driving modes to consider when using tow/haul mode towing a LighShip with an ICE vehicle.

Level Highways & Sustained Cruising: Leave It OFF (Normal Mode)​

On flat, open interstates, keeping Tow/Haul mode turned off is your best strategy for maximizing fuel economy. In normal driving mode, the truck’s transmission will upshift into its highest overdrive gears (e.g., 9th or 10th gear), dropping engine RPMs to an efficient cruising range. A standard high-profile trailer acts like a massive parachute, creating so much drag that an ICE truck cannot maintain highway speeds in its top overdrive gears without constantly downshifting ("gear hunting"). Because the compressed AE.1 slips through the air with minimal resistance, your engine can easily maintain a steady cruising speed in its highest, most efficient gears without straining. Forcing the truck into Tow/Haul here would lock out those top gears and waste fuel at unnecessarily high RPMs.

Since the majority of towing miles will be done in this scenario, ICE tow vehicles get a double boost in fuel efficiency towing with tow/haul mode off. You will see an 40% increase in fuel economy due to the aerodynamic efficiency of the LightShip. And you will see between a 5% to 10% (depending on ICE equipped tow vehicle) additional increase in fuel efficiency by not using Tow/Haul mode when it is not needed because the engine will be operating more efficiently in higher gears.

Ideally it would be best if the truck would watch these variables of trailer tow load and trailer aerodynamic drag, then automatically engage tow/haul mode when needed. But today, switching to tow/haul mode is a manual process by the driver based on driving conditions.

Level Highways & Sustained Cruising with Strong Head Winds: Tow/Haul Mode ON​

Strong sustained head winds progressively eliminate all the advantages of having an hyper aerodynamic trailer. During my trip yesterday between Page, AZ and Flagstaff, AZ, driving at 62 MPH, I had a long, flat desert drive with a 30 MPH headwind and gusts to 50 MPH. Drag increases with the square of the air speed. In this case, the truck is pulling the LightShip at an aerodynamic air speed of between 90 MPH to 110 MPH with a strong head wind with occasional gusts. Fuel efficiency plummets along with a high demand on the engine and transmission. During my trip yesterday, my fuel economy dropped from 17 MPG to 11 MPG in the high winds with tow/haul mode engaged.

Rolling Hills: The "Gear Hunting" (Flexible Usage)

In rolling terrain, thestrategy must pivot from aerodynamics to weight management. Start with Tow/Haul OFF. Watch the tachometer and listen to the engine. If the truck smoothly handles the rolling hills without constantly shifting back and forth, leave it off. However, if the transmission begins "hunting" (dropping two gears to climb a minor hill, upshifting at the crest, and immediately downshifting again on the next rise), toggle Tow/Haul ON. On an incline, aerodynamics matter less and physical mass matters more. The truck must pull all 8,400 pounds up the grade. Preemptively locking the torque converter via Tow/Haul eliminates fluid slippage and transmission heat buildup if the terrain gets too busy.

Mountain Passes & Steep Grades: Turn Tow/Haul Mode ON

When entering sustained mountain ranges, Tow/Haul mode should be engaged immediately. Gravity completely overrides aerodynamics on a steep grade. Your truck is hauling a heavy physical load up a mountain entirely on its own. Tow/Haul alters the shift mapping to hold lower gears longer, keeping the engine firmly in its optimal torque and power band while preventing the transmission from overheating due to constant shifting. This is purely a safety and vehicle-preservation tactic. The aerodynamic profile does very little to help you slow down a heavy trailer on a steep descent. Activating Tow/Haul commands the transmission to downshift aggressively when you tap the brakes, using engine braking to control your descent speed and preventing your truck’s friction brakes from overheating and fading.

Heavy Stop-and-Go Traffic: TurnTow/Haul Mode ON

Whenever you enter dense city traffic, tight construction zones, or stop-and-go gridlock, engage Tow/Haul mode. Aerodynamics mean nothing at 0 to 30 mph; inertia is the enemy here. Getting an 8,400-lb dead weight moving requires significant torque. Tow/Haul mode delays upshifts during acceleration, ensuring the truck stays in 1st, 2nd, and 3rd gears long enough to get the load moving smoothly. This reduces the mechanical strain and wear on your transmission's internal clutch packs caused by early, lazy upshifts.

The good news is that the vast majority of miles towing a LightShip will be on straight, fast, relatively level highways, with no wind or a tailwind. Tow/Haul mode should not be needed with a LightShip in tow with TrekDrive OFF under these conditions. As the headwinds increase, traffic slow downs happen or the grades begin to change at around 3%, the variables change requiring the use of Tow/Haul mode to protect the engine/transmission and ensure a safer drive.

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Bonus Content for all the Nerds - the impact on aerodynamic drag with Density Altitude

There is another variable to consider for everyone towing a trailer in the Western US that I did not include in the analysis above: Density Altitude. Aerodynamic drag is directly proportional to air density, meaning it decreases as density altitude increases. In the standard atmosphere, moving from sea level to 10,000 feet causes a significant drop in air density, which linearly reduces the drag force on an object at a constant true airspeed.

Here are the changes Between Sea Level and 10,000 Feet.

• Density Reduction: At 10,000 feet, air density is approximately 25% lower than at sea level.
• Proportional Drag Decrease: Because the drag equation, the drag force at a specific true airspeed also drops by about 25% over this altitude range.

This means for a turbocharged, ICE engine, equipped tow vehicle, demand on the engine/transmission drops significantly as density altitude (actual altitude as modified by ambient temperature) increases.

What all this means is that operating at altitude, on hot days, out West is yet another variable to consider when deciding if Tow/Haul mode is necessary or not. A higher density altitude will create less stress on the engine/transmission at a given higher speed than operating at the same speed at sea level.