A petroleum transport trailer holds roughly 9,000 to 9,800 gallons, split into four or five compartments by solid steel bulkheads. That split lets one truck carry regular, premium, and diesel on the same run, and it means every load is several products traveling together. The paperwork has to treat it that way.
What they are
Most fuel trailers are built with four or five compartments of different sizes, often between 1,000 and 3,000 gallons each. Each compartment carries its own product or grade: one might hold regular, the next premium, the next diesel. The bulkheads keep them separate from the rack to the customer, and they also cut down sloshing in transit.
A worked example
A common four-compartment layout splits 9,000 gallons as 3,000 + 2,000 + 2,000 + 2,000. Loaded for a typical station drop, it might look like this:
- Compartment 1 (3,000 gal): regular 87
- Compartment 2 (2,000 gal): regular 87
- Compartment 3 (2,000 gal): premium 93
- Compartment 4 (2,000 gal): diesel
That is one load on the road and four line items on paper. Each product has its own rack cost, its own selling price, and its own tax treatment, so the back office can never treat the trip as a single 9,000-gallon quantity.
Why trucks have them
So one run can serve different needs without dedicating a whole truck to a single product. A station usually needs more than one grade, and a route hits several customers. Compartments let a driver carry the right mix in one load, which makes dispatch and delivery far more efficient.
Loading at the rack
At the terminal rack, each compartment is loaded with its assigned product and metered separately, and the bill of lading records what went into each. That per-compartment detail is what the back office uses to bill, tax, and reconcile, since the load is several products at once.
Switch loading: why the separation is strict
The order of loads matters as much as the walls between them. Switch loading means filling a compartment with a high flash point product like diesel right after it carried gasoline. The Petroleum Equipment Institute flags two hazards: leftover gasoline vapor can contaminate the diesel, and the fill itself can build a static charge strong enough to ignite that vapor. Terminals manage it with bonding, slower fill rates, and product-order rules, and the BOL's compartment history tells the next loader what was in there.
Why it matters for accounting
A single load is multiple products, each with its own cost, price, and tax, and its own gross and net figures. Treat the truckload as one undifferentiated quantity and the numbers blur. Tracking by compartment and product keeps cost, margin, and fuel tax accurate for each grade. FastDragon Fuel Jobber carries each compartment from the BOL through billing, tax, and inventory, so the 9,000-gallon load above bills as four separate line items.
Frequently asked
Why don't drivers fill every compartment to the top?
Weight usually limits the load before volume does. The federal gross weight limit on interstates is 80,000 pounds, and diesel weighs about 7 pounds per gallon versus roughly 6.1 for gasoline, so a diesel-heavy load maxes out sooner. Drivers also leave outage, headspace that lets the fuel expand as it warms.
What is a retain in a tanker compartment?
A retain, sometimes called a heel, is product left in a compartment from a previous load. Terminals and carriers record retains because even a small amount of gasoline left behind can put the next diesel load off spec. Many carriers require an empty, verified compartment before a product change.
What happens if the wrong compartment gets dropped at a station?
That is a cross-drop: gasoline into a diesel tank or the reverse. The contaminated station tank has to come out of service and get pumped down before anyone sells from it, which can idle dispensers for a day or more. Matching compartment numbers to drop points on the delivery ticket is the standard guard against it.
Does each compartment have its own valves and piping?
Yes. On a typical petroleum trailer each compartment has its own internal emergency valve, discharge outlet, and vapor connection, so it loads and unloads independently of the others. That independence is what lets a driver drop the diesel compartment at one stop and carry the gasoline on to the next.