Everybody who sells trucks in Montrose has watched the same thing happen. A customer tows fine at home, points the rig at a pass, and finds on the climb that the truck feels like a different vehicle. Then finds something worse on the descent. It is not a defect. It is physics, and it is predictable.

How much power does a truck lose at altitude?

An engine makes power by burning fuel with oxygen. Thin air holds less oxygen per cubic foot, so a naturally aspirated engine, one with no turbocharger, gets less of what it needs the higher it goes.

Garrett Motion's working figure is a loss of about 3% of power per 1,000 feet. A 100 hp engine makes around 85 hp at 5,000 feet. Ford has been quoted at 3 to 4% and Nissan around 4%, so it varies by who is measuring.

Run that out. At around 11,000 feet, a naturally aspirated gas engine loses roughly a third of its power. Ram's 6.4L HEMI in an HD makes 405 hp and 429 lb-ft at sea level. Near the top of a pass, you are pulling with substantially less than that.

Why does a turbocharged engine handle altitude better?

Because a turbocharger does not wait for the atmosphere to supply air. It compresses whatever air is available, raising boost as ambient density falls to reach the same manifold pressure. The compensation isn't perfect, but it is significant, and it is the most important mechanical difference between a gas HD and a diesel HD.

Ram's 6.7L Cummins High Output makes 430 hp and 1,075 lb-ft, and keeps far more of that at elevation than a naturally aspirated gas engine keeps of its own. GM's Duramax L5P, at 470 hp and 975 lb-ft, behaves the same way. That is the honest diesel argument: not that it tows more on paper, but that it still makes its power where you actually drive.

Does any manufacturer publish an altitude derate?

One does, and it is not us. Ford is the only one of the three that publishes a formal altitude derate, quoted exactly:

"While driving at elevation, in order to match driving performance as perceived at sea level, reduce Gross Vehicle Weight (GVW) and Gross Combination Weight (GCW) by 2% per 1,000 ft. elevation."

At 10,000 feet, that is a 20% reduction, and because it applies to gross vehicle weight, it derates payload too. GM publishes no altitude statement in any heavy-duty trailering document we checked, and Ram publishes no derate either, though no 2027 Ram towing guide exists yet, so treat that as unconfirmed rather than settled.

Policy as published in September 2026. "None published" means we found no statement, not that the manufacturer disputes the physics. Confirm against your own owner's manual and towing guide.

Here is the part that matters more than the badges. The physics applies to every truck on this lot and every truck on anyone else's. One manufacturer writes it down. That does not make Ford's trucks weaker or Ram's stronger. Ford handed you a planning rule, and the other two left you to work it out. Use it whatever you drive, and if it says the rig is too heavy, believe it.

What is an exhaust brake and why do diesels need one?

Everything above is about going up. The descent is where people get hurt.

An exhaust brake is a butterfly valve in the exhaust that closes when you lift off the throttle. With the exhaust sealed, the engine pumps against heavy backpressure on every stroke. That work comes from the forward motion of your truck and trailer, so the rig's kinetic energy becomes heat in the engine instead of in your brakes.

Diesels need the valve because they have no throttle plate. A gas engine, lifted off, is already pumping against a nearly closed throttle, and that restriction is what you feel as engine braking. A diesel breathes freely and produces very little engine braking on its own. The exhaust brake gives it back.

Ram's Cummins comes with a standard Diesel Exhaust Brake and a standard integrated trailer brake controller, confirmed on a 2027 window sticker. Credit where it is due: GM makes its trailer brake controller standard on every 2026 Silverado HD and Sierra HD trim too, and its Duramax reportedly brakes through the variable-geometry turbo, though we have not verified that with GM.

What is brake fade, and why is it the failure mode that matters?

Friction brakes turn motion into heat. On a long descent with a trailer pushing you, there is a lot of motion to convert and nowhere for the heat to go.

Past a certain rotor temperature, the pad material begins to vaporize. That gas forms a thin layer between the pad and rotor; the pad stops gripping, and pedal pressure stops producing stopping force. That is brake fade. The pedal still moves. The truck does not slow. It gives no warning, and it arrives at the worst point on the hill.

The point of the exhaust brake is that the friction brakes never get near that temperature because the engine does the work instead.

How do you actually tow a pass?

• Gear down before the descent, not during it. Select your gear at the top while your brakes are cold and you still have options. Starting down first and trying to recover your speed later is how the trouble starts.
• Turn the exhaust brake on and let it carry the load. It is built to hold your speed on a grade. Riding the pedal to help it heats the brakes you are trying to protect.
• Watch transmission temperature on the climb. Going up is a heat problem too, just a quieter one. Tow/haul mode keeps the transmission from hunting between gears and cooking itself.
• Assume you have less engine than the brochure says. Plan around a naturally aspirated engine losing roughly 3% per 1,000 feet, and give a turbocharged one credit for holding most of its output.
• Check payload before you check the tow rating. Payload is gross vehicle weight rating minus what your truck weighs, and it covers people, gear, hitch hardware, and the trailer's tongue (pin) weight. GM puts tongue weight at 10 to 15% of trailer weight and pin weight at 15 to 25%. Ford uses 10% and 15%. Run it at the wider band, and the yellow sticker on your door jamb runs out long before the tow rating does.

The verdict

If you tow heavy overpasses more than a few times a year, buy the turbodiesel. Not for the tow rating, but because it still has its power at the top of the hill and a real engine brake on the way down. A Ram 2500 or 3500 with the Cummins High Output and its standard exhaust brake is built for the roads out of this valley.

If you tow light and mostly at valley elevation, the 6.4L HEMI is a lot of truck for less money, and there is no shame in that math. Just know which truck you bought before you point it at Red Mountain with a full trailer.