Heavy-duty trucks like the Cummins-powered Ram, Duramax-powered Silverado/Sierra, and Powerstroke-powered Super Duty get surprisingly good MPG numbers — especially on the highway.
But why?
How does a gigantic diesel pickup weighing 8,000+ pounds get better fuel economy than a smaller gas truck?
It’s not magic. It’s science.
Diesel Fuel Contains More Energy
The biggest reason diesel trucks get better MPG starts with the fuel itself.
Diesel fuel contains more energy per gallon than gasoline.
On average:
- Gasoline → ~114,000 BTUs per gallon
- Diesel → ~129,000 BTUs per gallon
That means diesel fuel simply packs more usable energy. More energy = more work per gallon consumed.
Diesel Engines Are More Thermally Efficient
This is where things get nerdy — but also really interesting.
Diesel engines operate at much higher compression ratios than gasoline engines.
Typical examples:
- Gas engine → ~10:1 compression ratio
- Diesel engine → ~16:1 to 20+:1 compression ratio
Higher compression allows diesel engines to extract more energy from the fuel during combustion.
Simple version: diesel engines waste less energy.
That improved thermal efficiency is a huge reason diesel trucks can travel farther on less fuel.
Diesels Make Massive Torque at Low RPM
This is another major factor.
Diesel engines produce huge amounts of torque at very low RPM.
For example, modern version of the:
- 6.7 Cummins
- 6.6 Duramax
- 6.7 Powerstroke
All make 800+ lb-ft of torque without even trying hard.
That means the engine doesn’t need to rev high to move heavy loads.
Lower RPM = lower fuel consumption. That’s especially noticeable while towing.
Turbocharging Helps Efficiency Too
All modern diesel trucks rely on turbochargers.
Turbochargers help improve efficiency by:
- Drastically increasing airflow into the engine
- Improving combustion efficiency
- Harnessing waste energy (exhaust gas)
Instead of needing a huge naturally aspirated engine, turbo diesels can make big power efficiently.
Why Diesels Excel at Highway MPG
Diesel trucks really shine on the highway.
At cruising speeds, diesel engines:
- Operate at low RPM
- Stay within peak efficiency ranges
- Use torque instead of RPM to maintain speed
That’s why it’s not uncommon to see:
- 20+ MPG from modern heavy-duty diesels, especially with aftermarket tuning
- Even better numbers from smaller diesels like the 3.0 Duramax
Which honestly sounds ridiculous when you think about how large these trucks are.
So Why Doesn’t Everyone Drive Diesels?
Great question.
Diesels have downsides too.
- Higher upfront cost
- More expensive maintenance
- Complex emissions systems on trucks made after 2007
- Diesel fuel is typically more expensive than gasoline
Modern diesel trucks are incredibly capable — but they’re usually more expensive to own and repair than most gas trucks.
The Bottom Line
The simple version of a very long-winded conversation is that diesel trucks get better MPG because:
- Diesel fuel contains more energy
- Diesel engines are more thermally efficient
- They produce high torque at low RPM
- Turbocharging improves efficiency
It’s a combination of engineering, physics, and fuel chemistry.
Have questions? We're here to help.
- Phone: 636-429-7020
-
Email: info@fassmotorsports.com
FASS Motorsports
25 Town and Country Drive
Washington, MO 63090
