Diesel transfer tanks: sizing, legal limits, and setup

Written by a fuel-system technician. Updated October 28, 2026.

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A transfer tank is a fuel tank mounted in a pickup bed or on a trailer that you pump from to fuel equipment in the field. For diesel, keeping the tank at 119 gallons or less keeps it out of most federal hazmat rules. Over 119 gallons counts as "bulk" packaging, and diesel in bulk packaging needs placards (J.J. Keller).

Gasoline is a different story: it's regulated much more tightly, even in small tanks. This guide covers the rules, choosing a size your truck can carry, and setting it up right.

The rules for hauling fuel in a pickup

Under federal hazmat rules, what matters is the fuel and the tank's capacity, not how much is in it. A 120-gallon tank with 20 gallons in it is still a bulk package (J.J. Keller).

Federal hazmat rules for diesel. Gasoline is regulated more tightly; see the table below. States can add their own rules.

Situation	What the federal rules say
Diesel in a tank of 119 gallons or less	Diesel with a flash point of 100°F or higher can be treated as a combustible liquid, and the hazmat rules don't apply to combustible liquids in non-bulk packaging. No placards, labels, or shipping papers (J.J. Keller; PHMSA letter 21-0001)
Diesel in a tank over 119 gallons	Bulk packaging. Placards are always required for diesel in bulk packaging (J.J. Keller)
Two or more diesel tanks connected together	If fuel can flow between them, PHMSA treats them as one bulk package. Keep the valves between them closed during transport (PHMSA letter 21-0001)
Gasoline in small containers	As "materials of trade" used in your business, up to 8 gallons per container and 440 pounds total on the vehicle (49 CFR 173.6)
Gasoline in a bed-mounted tank	Generally needs a tank built and used under a DOT special permit. For example, Transfer Flow's permit covers tanks up to 119 gallons, requires the pump to be disconnected from power in transit, and bans connecting tanks together (DOT-SP 11911)

Farms get an extra exception. Farmers hauling fuel to or from the farm within 150 miles, intrastate, can carry up to 502 gallons of liquid agricultural products per vehicle, including diesel and gasoline, with relief from some packaging, training, and emergency response rules (PHMSA agricultural brochure). Your state's rules still apply.

Check the label before you buy a tank for gasoline. Some tanks are sold as approved for diesel, gasoline, and aviation fuel (RDS 60-gallon combo). A tank sold for diesel only shouldn't carry gasoline.

Choosing a tank

Start with your truck's payload, not the biggest tank that fits. Diesel weighs about 6.9 pounds per gallon (EPRI), so fuel adds up fast:

Tank size	Weight of the fuel alone, full
50 gallons	About 350 lb
75 gallons	About 520 lb
100 gallons	About 690 lb
119 gallons	About 830 lb

Add the empty tank, pump, toolbox contents, and anything else in the bed, and compare the total to the payload on your door sticker. A half-ton pickup can run out of payload before it runs out of bed space.

Style	Good for	Trade-off
Rectangular, against the cab	Most capacity for the footprint	Uses bed space you might want for cargo
L-shaped	Leaves more open bed floor beside the tank	Less capacity for the same bed length
Combo tank with built-in toolbox	Fuel and tool storage in one unit, often with a lockable fill cap and level gauge (RDS 95-gallon L combo)	Heavier and costs more than a plain tank

Material: Aluminum tanks don't rust and weigh less (RDS 95-gallon L combo). Steel tanks usually cost less but add weight and can rust if the coating is damaged.

Features worth having: internal baffles to keep fuel from sloshing, a lockable fill cap, a rollover vent, a drain plug at the bottom, and a level gauge (RDS 60-gallon combo). The drain plug matters more than people think: it's the easiest way to get water out (see water in your diesel tank).

Shopping: RDS combo tanks on Amazon, L-shaped transfer tanks on Amazon, and Fill-Rite pump kits on Amazon. Check that the tank is labeled for the fuel you'll carry.

Setting it up

Bolt the tank down, put a filter after the pump, bond the pump to the tank, and cut power to the pump when you drive.

Mount it solid. Use the manufacturer's mounting hardware and bolt through the bed, or use tie-downs rated for the full weight. A loose tank full of fuel is a hazard in a hard stop.
Pick a pump that fits. For most pickup transfer tanks, a 15 GPM 12V pump is the usual fit. See how to choose a 12V fuel transfer pump for sizing and wiring.
Wire it to the battery with a fuse. Fill-Rite calls for at least 12-gauge wire and a 30-amp fuse near the battery on its 12V pumps (Fill-Rite manual).
Bond and ground. Connect the pump to the tank or truck frame with its bonding wire, and use a hose with a static wire (Fill-Rite manual; Fill-Rite FR4210H).
Add a filter after the pump. It keeps dirt and water out of the equipment you fuel. A 10-micron water-detecting filter is a good default (Fill-Rite filters).
Keep the suction tube off the bottom. At least 3 inches up, so it doesn't pull water and sediment (Fill-Rite manual).
Shut it down for the road. Turn off the pump, secure the nozzle, and for a gasoline tank under a special permit, disconnect power and keep the hose and nozzle below the permit's "safe zone" (DOT-SP 11911). It's a good habit for diesel too.

Keeping the fuel clean

Transfer tanks spend a lot of time half empty and out in the weather, which is exactly how water gets into fuel. Long stretches at a low level cause condensation, and caps, gaskets, and vents that don't seal let in rain and snow (CRC Report 672).

Check the bottom with water-finding paste every month or so, and drain any water from the bottom plug.
Keep the fill cap locked and the gasket in good shape.
Refill before the tank sits low for weeks, especially going into winter.
In cold weather, a pickup tank is as exposed as any aboveground tank. Treat or blend the fuel early (see diesel gelling).

FAQ

Do I need placards for a 100-gallon diesel transfer tank?

Under federal hazmat rules, diesel in a tank of 119 gallons or less doesn't need placards (J.J. Keller). Whether you need a CDL depends on your truck's weight rating and use, not just the tank. Check your state's rules.

Can I put two 100-gallon tanks in my truck?

If they're connected so fuel can flow between them, PHMSA treats them as one bulk package, which brings placards. Keep the valves between them closed in transit (PHMSA letter 21-0001).

Can I haul gasoline in my transfer tank?

Only if the tank is approved for gasoline. Bed-mounted gasoline tanks are built under DOT special permits with conditions such as no connecting tanks together and cutting power to the pump in transit (DOT-SP 11911).

Does a half-full 150-gallon tank count as 75 gallons?

No. The rules go by the tank's capacity, not what's in it (J.J. Keller).

Sources

This guide is general information, not legal advice. Hazmat and vehicle rules vary by state and by how the truck is used; check with your state DOT for your situation.