Auxiliary Fuel Tank Capacity and Sizing | Know Your Limits

Auxiliary fuel tank capacity ranges from 30 gallons for trucks to 10,000+ gallons for industrial use, limited by space, weight, state rules.

For the full breakdown, see our best Auxiliary Petrol Tank guide.

An auxiliary fuel tank adds extra fuel storage beyond your factory tank, extending range for long hauls, remote work sites, or emergency preparedness. Getting auxiliary fuel tank capacity and sizing right means the difference between a seamless installation and a costly mistake. Choose too small and you are still making frequent fuel stops; too large and you waste payload capacity and risk legal trouble.

Typical Capacities by Application

Auxiliary fuel tanks span a wide range of sizes based on the vehicle type and use case. Smaller truck tanks fit in the bed, while industrial installations are often bulk storage tanks on a concrete pad. The table below outlines the most common categories.

For pickup trucks, in-bed auxiliary tanks are commonly available from 30 to 100 gallons. Steel rectangular models come in 40, 50, 75, and 100-gallon sizes from brands like Transfer Flow, while aluminum options include precise capacities like 30, 41, 42, 43, and 46 gallons through brands like ATI. Each size corresponds to specific dimensions.

What Size Auxiliary Fuel Tank Does Your Truck Need?

Start by measuring your truck bed or mounting area in three dimensions: length, width, and height. Leave room for toolboxes, straps, valves, filler necks, and access around the tank. An installation example for a 2018 Ram 3500 shows a tank about 59.5 inches long with roughly 2.5 inches of clearance on each side for mounting brackets. If your bed has a liner or toolboxes, account for those too — every inch matters when fitting a large tank.

Fuel weight is the next big constraint. Compare that total against your vehicle’s payload rating — exceeding this limit affects handling and braking.

Plan for 10–15 percent extra space in your mounting area for hose routing, filler neck placement, and natural bladder expansion over temperature changes. Also understand that a tank’s usable capacity is always less than its nominal capacity — a 275-gallon tank may hold only 250–256 gallons of usable fuel because of required head space for expansion and safe fill limits. Always use the usable capacity, not the nominal figure, when calculating your actual range extension.

For commercial bulk installations, size the tank based on 3 to 4 weeks of typical usage between deliveries. Review fuel invoices to calculate your average consumption. Taking these factors into account before you buy ensures the tank fits physically, legally, and within your vehicle’s weight limits.

Regulatory and Safety Limits

State regulations set hard capacity ceilings that vary across the country. Illinois limits auxiliary fuel tanks to 119 gallons aggregate capacity for Class I or II liquids on ½-ton and ¾-ton pickup trucks or larger vehicles under Illinois Administrative Code title 41, section 174.430. This total includes fuel in the factory tank as well.

The tank itself must meet construction standards — Illinois requires 18-gauge steel or equivalent-gauge aluminum, with secure fastening so it cannot separate during a collision.

Also understand the distinction: an auxiliary tank feeds your engine by transferring fuel to the factory tank, while a transfer tank is primarily for storage and transport. This affects which pumps, valves, and plumbing you need. If you want a tank that directly powers your truck, choose an auxiliary system, not a transfer tank. Diesel and gasoline systems also differ in fittings and compatibility, so match the tank to your engine type.