The 80,000-pound federal weight limit usually caps how much fuel a tractor-trailer tanker can carry before the tank is full.
Tank size varies by build, so check the capacity plate on the trailer in front of you. The rest of this post is the arithmetic that turns tank size into a load you can legally carry, using a 9,000-gallon trailer as the example.
What actually limits how many gallons a fuel tanker can carry?
The federal gross vehicle weight limit does. Under 23 CFR 658.17, which applies to the Interstate System and reasonable access to it, the maximum gross vehicle weight is 80,000 pounds, except where a lower weight is dictated by the bridge formula. Single axles are capped at 20,000 pounds and tandem axles at 34,000 pounds. Fuel is heavy enough that the truck can hit those limits before the tank is full.
So the load is a subtraction: 80,000 pounds, minus what the empty tractor, trailer and driver weigh, leaves the pounds of fuel you can carry. Divide by the weight of a gallon and you have gallons. Some states and routes have their own rules, so treat 80,000 as the federal ceiling and check your lanes.
How much does a gallon of gasoline or diesel weigh?
Gasoline weighs roughly 6 pounds a gallon and diesel roughly 7. You get there from specific gravity, which is a fuel’s density compared with water. Water is 62.424 pounds per cubic foot at its densest, per the USGS, and a U.S. gallon is 231 cubic inches, so a gallon of water is about 8.34 pounds.
The fuel figures come from safety data sheets, and they are ranges because fuel varies by blend, season and temperature:
- A CountryMark unleaded gasoline sheet lists specific gravity at 60 °F of 0.7 to 0.78.
- A Sinclair No. 2 diesel sheet lists relative density of 0.82 to 0.88.
The table uses the midpoint of each range. The midpoints are an illustration, not a spec for any product you load.
| Fuel | Specific gravity (midpoint of SDS range) | Pounds per gallon (calculated) |
|---|---|---|
| Gasoline | 0.74 | 6.18 |
| Diesel | 0.85 | 7.09 |
Terminal paperwork often states volume corrected to a standard temperature (net gallons), which is why the gallons on a ticket and the gallons in the tank can differ. See gross vs. net gallons.
How many gallons can one tanker legally haul? A worked example
Say the tractor, an empty tank trailer and the driver together weigh 33,000 pounds. That tare weight is illustrative, not a customer or manufacturer figure; yours will differ with the tractor, the tank material and the fuel on board. It leaves 47,000 pounds for product.
| Product | Pounds per gallon | Payload (80,000 - 33,000) | Gallons at the weight limit |
|---|---|---|---|
| Gasoline | 6.18 | 47,000 lb | about 7,600 |
| Diesel | 7.09 | 47,000 lb | about 6,600 |
All inputs are illustrative. The pattern holds regardless: the same truck hauls roughly 1,000 fewer gallons of diesel than gasoline. Across the SDS ranges, the gasoline answer moves between about 7,200 and 8,000 gallons for this tare, and diesel between about 6,400 and 6,900.
Two things follow:
- A trailer that is 9,000 gallons on paper may leave about 1,400 gallons of tank empty on a gasoline load and about 2,400 on diesel, if the truck is at the weight limit in this example.
- A lighter tractor, trailer or tank material buys you gallons. Every 1,000 pounds of tare is roughly 160 gallons of gasoline or 140 of diesel.
Axle weights matter too. A load that is under 80,000 pounds in total can still put too much on one axle group, depending on where the product sits.
Why does gallons per load change your cost per gallon?
Because when freight is paid as a flat amount per load, fewer gallons on the truck means a higher cost on every gallon. Run the diesel load in the example above at 6,600 gallons instead of the 7,600 gasoline gallons and the same flat rate spreads over about 13 percent fewer gallons.
Fuel hauling rates from per mile to per gallon shows the conversion. When you price a lane, price it for the product you will carry. If you compare bulk fuel quotes, check which gallon count each quote assumes.
How do compartments change what a tanker carries?
A tank trailer is divided into compartments so one truck can carry different products or deliver in separate drops. Most fuel trailers have several; the count and sizes vary by build, so confirm yours from the trailer’s spec plate. Each compartment is sized and filled on its own, with a hatch and its own outlet.
Compartments let you run a split load: gasoline in some, diesel in others, or one product delivered to two customers. The weight math above still applies to the whole truck, so a split load of both products lands between the two gallon counts. A truck with 4,000 gallons of gasoline and 3,000 of diesel weighs, in product, about 24,700 plus 21,300 pounds, which is 46,000 pounds. That is under the 47,000 in our example. On the 9,000-gallon trailer from earlier, about 2,000 gallons of space is left.
Compartments also mean you cannot just divide by total volume. The sizes have to match the drops. Hazmat paperwork rides with all of it; see hazmat placards and shipping papers for fuel.
What about smaller fuel trucks?
Smaller delivery trucks, often called bobtails or tank wagons, carry tanks smaller than a tractor-trailer tank, with sizes that vary by build. They are usually limited by the same weight logic at a lower gross rating.
They carry less per trip but reach places a full trailer cannot, which is why they are common for farm, home and small-site deliveries. Rates for them may be quoted differently from full-load rates; the formula for cost per gallon is the same, with fewer gallons on the bottom.
If you are sizing a business around one of these, how to start a fuel hauling business covers the steps in order, from authority and insurance to driver endorsements and terminal access, and fuel tanker load boards covers where the loads are posted.
How Energy Connect handles it
Gallons per load drives the freight rate you bid. On Energy Connect, carriers can bid on a posted load, and the poster can accept or decline each bid. A bid quoted as a freight rate can also be countered; a bid priced off an index or terminal can only be accepted or declined.
Once the load is moving, you record dispatch, pickup, transit and delivery on the load, see pickup, delivery and verification times on it, and attach the bill of lading and other delivery documents to it.