Why do diesel trucks have two gas tanks?

In most modern diesel trucks, you’ll see one main diesel fuel tank and a separate tank for diesel exhaust fluid (DEF). A second fuel tank is not common for every model and is usually an optional feature on certain heavy-duty configurations aimed at extending range or meeting fleet needs.

What are the two tanks and what do they do?

Here is a quick breakdown of the tanks you’ll typically encounter in today’s diesel trucks and what each one does. This helps distinguish between what's a fuel tank, what's an emissions-related tank, and when a second fuel tank might appear.

• Primary diesel fuel tank – Stores the fuel that actually powers the engine. The capacity varies by model but is generally in the 20–40+ gallon range for light-to-heavy-duty trucks.


• DEF (Diesel Exhaust Fluid) tank – Stores a urea-water solution used by the selective catalytic reduction (SCR) system to reduce nitrogen oxide (NOx) emissions. This is not a fuel and is essential for compliant operation on modern diesel engines. DEF tanks are typically smaller, often in the 5–8 gallon range, but capacities vary.


• Optional secondary diesel tank – Some heavy-duty and fleet configurations offer a second diesel tank to extend driving range or balance weight and plumbing in challenging duty cycles. This is not standard on most pickups and is more common on commercial or specialty builds.

In practice, the DEF tank is mandatory for emission compliance in most regions, while a second diesel tank is an optional arrangement that only appears on certain models or configurations. The two-tank setup you encounter is therefore usually “diesel fuel + DEF” plus, in some cases, an extra primary fuel tank.

Why DEF matters in diesel engines

What is DEF and why is it required?

DEF stands for Diesel Exhaust Fluid, a non-toxic, non-hazardous solution of urea and deionized water. It is injected into the exhaust stream before the SCR catalyst to chemically reduce NOx emissions, turning harmful compounds into nitrogen and water. Modern diesel engines rely on DEF to meet increasingly stringent emissions standards in many markets. Without sufficient DEF, the engine may reduce power or enter a limp mode to protect the aftertreatment system.

DEF is consumed during normal operation and must be replenished periodically. Running dry can trigger warning messages and potential performance limitations, so fleet managers and individual owners keep an eye on DEF levels just as they do with fuel. The DEF tank is never used as a fuel source; it has its own refill interval separate from the diesel tank.

Two-tank configurations in practice

Some trucks are built with two tanks to address specific use cases. The following scenarios explain why two tanks might appear on a diesel vehicle.

• Extended driving range for long-haul or remote operations — A second diesel tank can allow more fuel capacity, reducing refueling stops on long trips or in areas with sparse service stations.


• Redundancy or easier fueling logistics in fleets — Fleets operating in harsh environments may opt for dual tanks to simplify maintenance, fuel management, and vehicle uptime.


• Historical or specialty applications — In older diesel designs or specialized chassis configurations, dual tanks were more common to accommodate biodiesel blends, fuel distribution layouts, or ballast considerations.


• Weight distribution and fitment constraints — In certain heavy-duty applications, a second tank may be placed to achieve more favorable weight balance or to fit within a particular frame design.

Not every diesel truck offers a second diesel tank, and many buyers today will see a single primary fuel tank plus a dedicated DEF tank. When a second diesel tank is present, it is typically part of a heavy-duty or commercial-spec configuration rather than a feature on typical consumer pickups.

Maintenance and operation considerations

Understanding how the two-tank setup works helps with maintenance, fueling habits, and avoiding problems on the road.

• Fueling order and separation – Always fuel with diesel from the designated diesel tank. DEF is refilled as needed from its own fill point. Do not confuse the two or add DEF to the diesel tank.


• DEF management – Monitor DEF levels through the vehicle’s dashboard or telematics. Refill DEF before it runs dry to avoid performance restrictions or warning lights.


• Second diesel tank considerations – If your vehicle has a second diesel tank, know which tank is active, how the switch works (some vehicles do not require manual switching), and how to monitor levels for both tanks.


• Emissions system upkeep – Keeping the SCR system functioning (including DEF quality and dosing) is essential for durability and compliance. Poor DEF quality or low DEF can lead to increased NOx emissions and potential drivability issues.

For most owners, the primary takeaway is that diesel engines rely on diesel fuel and DEF for emissions control. The presence of two diesel tanks is less common and usually tied to specialized applications, while the DEF tank is a standard feature across modern diesel powertrains in many markets.

Summary

Diesel trucks typically use one main diesel fuel tank and a separate DEF tank to support emissions-compliant operation. A second diesel tank is not universal and tends to appear in specific heavy-duty or fleet configurations aimed at extending range or meeting particular duty-cycle requirements. DEF plays a central role in reducing NOx emissions, while the second fuel tank, when present, serves less common purposes tied to range, balance, or specialized applications. For most drivers, the day-to-day experience centers on managing diesel fuel and DEF rather than juggling two full diesel tanks.

Why does the F250 have two gas tanks?

To today's motorists an older Ford F-250 with two fuel filler doors on its flank might be an odd sight, but decades ago this wasn't unusual for pickup trucks. Yes there actually were two fuel tanks behind those body panels, and the extra fuel capacity was there to increase the range of these workhorses.

What happens if I put 2 gallons of gas in my diesel truck?

You should not drive the vehicle and immediately have the tank drained and the fuel lines flushed to avoid engine damage. While a small amount of gasoline in a diesel tank might not cause immediate catastrophic failure, it will damage expensive components like the fuel injectors and fuel pump over time. 
Immediate actions

• Do not start the engine. If you have not started it, do not turn the key. If you have already started it, pull over and turn the engine off immediately. 
• Have the vehicle towed. Do not drive the vehicle to a mechanic. It needs to be towed to a service center for a full fuel system flush. 

This video shows what happens when you accidentally put gas in a diesel engine: 57sBig Truck Big RVYouTube · Apr 30, 2017
What to expect from a mechanic

• Fuel tank and line flush: The mechanic will drain the tank completely and flush the fuel lines to remove all traces of gasoline. 
• Fuel filter replacement: The fuel filter will need to be replaced, as the gasoline can clog it. 
• Potential component replacement: Depending on the amount of time the engine ran, the fuel injectors and fuel pump may need to be replaced due to lack of lubrication and damage from gasoline combustion. 

Why it's a problem

• Lack of lubrication: Diesel fuel acts as a lubricant for the high-pressure fuel system components. Gasoline lacks these lubricating properties and can cause significant wear and damage to the fuel pump and injectors. 
• Combustion issues: Gasoline ignites at a different pressure and temperature than diesel, which can lead to improper combustion, knocking, stalling, or other performance issues. 

What is the purpose of the auxiliary fuel tank?

AUXILIARY FUEL TANKS are defined by the Vehicle Equipement Safety Commission as an additional fuel tank and any other componets attached directly thereto designed to supplement the vehicle's fuel carrying capability beyond that provided by the vehicle manfacturer.

What is the purpose of a double wall fuel tank?

A double-wall tank has an interstitial space between the two walls that fulfills the function of secondary containment in the event of leakage. No external containment safety feature is required in the storage site.