1.3 Product Density, Legal Weight Limits & How Much to Load
Key Takeaways
- The AAMVA manual says the amount to load depends on the liquid's expansion in transit, the weight of the liquid, and legal weight limits.
- On the Interstate System, 23 CFR 658.17 sets 80,000 lbs gross, 20,000 lbs on a single axle, and 34,000 lbs on a tandem, subject to the bridge formula.
- Water weighs about 8.34 lbs per gallon, gasoline about 6.1, and concentrated sulfuric acid about 15.3, so the same tank holds very different legal loads.
- A full tank of a dense liquid such as some acids can exceed legal weight limits, so dense products are often hauled as partial loads.
- Too much weight on the steering axle causes hard steering, while too little makes steering unsafe and too little on drive axles causes poor traction.
A tank that holds 9,000 gallons does not mean 9,000 gallons is a legal or safe load. Liquids differ in weight per gallon, and legal limits cap what the vehicle may weigh. The AAMVA manual's section on how much to load, and the federal requirement to understand surge with liquids of different densities, both come back to this math.
The Three Factors (AAMVA Manual 8.2.7)
The manual says a full tank of dense liquid, such as some acids, may exceed legal weight limits, so you may often only partly fill tanks with heavy liquids. The amount to load depends on:
- The amount the liquid will expand in transit (outage, Section 3.1)
- The weight of the liquid (density)
- Legal weight limits
Density: Weight per Gallon
Cargo weight is volume times density: cargo weight (lbs) = gallons x pounds per gallon.
Water at about 60 degrees F weighs about 8.34 lbs per gallon. Products lighter than water weigh less per gallon, and heavier ones weigh more. Shippers give exact density for each product. The values below are approximate and change with temperature and concentration.
| Product | Approx. lbs per gallon | Gallons in a 50,000-lb payload |
|---|---|---|
| Gasoline | 6.1 | about 8,200 |
| Ethanol | 6.6 | about 7,600 |
| Diesel fuel | 7.1 | about 7,000 |
| Water | 8.34 | about 6,000 |
| Milk | 8.6 | about 5,800 |
| 28% UAN liquid fertilizer | 10.6 | about 4,700 |
| 50% sodium hydroxide (caustic soda) | 12.7 | about 3,900 |
| 93-98% sulfuric acid | 15.3 | about 3,270 |
Federal Weight Limits
On the Interstate System and reasonable access routes, 23 CFR 658.17 sets:
- 80,000 lbs maximum gross vehicle weight, unless the bridge formula requires less
- 20,000 lbs on any one axle
- 34,000 lbs on a tandem axle
- The Bridge Gross Weight Formula for groups of axles
States set limits for other roads and issue permits, so your carrier's routing and state rules also matter. Tank nameplates add limits of their own: the specification plate lists maximum payload and the nameplate lists maximum design density of lading (Section 6.2).
Payload Math
Allowable payload = legal gross weight - empty (tare) weight of the loaded rig, where tare includes the tractor, trailer, fuel, and driver.
Example 1: Maximum gallons of a dense product
A tractor-tanker has a tare weight of 29,500 lbs and will haul caustic soda at 12.75 lbs per gallon.
- Allowable payload: 80,000 - 29,500 = 50,500 lbs
- Maximum gallons: 50,500 / 12.75 = about 3,960 gallons
- Loading 4,500 gallons would weigh 57,375 lbs and bring the gross to 86,875 lbs, which is overweight.
Example 2: Same tank, different products
A 9,200-gallon tank trailer with a 28,000-lb tare:
| Product | Load | Cargo weight | Gross weight | Result |
|---|---|---|---|---|
| Gasoline | 8,500 gal at 6.1 | 51,850 lbs | 79,850 lbs | Legal gross |
| Water | 9,200 gal at 8.34 | 76,728 lbs | 104,728 lbs | About 24,700 lbs over |
| Water | 6,200 gal at 8.34 | 51,708 lbs | 79,708 lbs | Legal, but tank only about 67% full |
A tank rated for 9,200 gallons can be legally full of gasoline, while water has to be loaded as a partial load. The tank must also be approved for the product; the nameplate's maximum lading density and material compatibility rules decide that.
Density and Surge (49 CFR 383.119(e))
The federal knowledge list includes differences in cargo surge for liquids of different densities. Two ideas explain it:
- Dense products usually go as partial loads. At a legal weight, a dense liquid fills less of a large tank. The larger empty space gives the liquid more room to move, so surge in a partial load is more of a problem than in a nearly full tank.
- Each gallon of a dense liquid is heavier. When the same volume of liquid sloshes, a heavier product carries more momentum into the tank head or wall.
This is why dense products are often hauled in smaller-capacity tanks built for them, so a legal load fills more of the tank.
Weight Balance and Axle Loads
Legal gross weight is not enough. The AAMVA manual's cargo section warns:
- Too much weight on the steering axle can cause hard steering and damage the steering axle and tires.
- Under-loaded front axles (weight shifted too far to the rear) can make the steering axle weight too light to steer safely.
- Too little weight on the drive axles can cause poor traction; drive wheels may spin easily, and in bad weather the truck may not be able to keep going.
- A high center of gravity increases the chance of rollover.
For multi-compartment tanks, this becomes a loading and unloading plan (Section 2.3). Use scales and the carrier's loading charts, not guesses.
Common Test Traps
- "The tank holds it, so it's legal." A full tank of a dense liquid can be overweight.
- "Lighter is safer." A legal partial load of a dense product surges more than a nearly full tank.
- "Fuel is heavier than water." Gasoline and diesel weigh less per gallon than water.
A tractor-tanker has a tare weight of 30,000 lbs and will load an acid weighing 15.0 lbs per gallon. Ignoring axle limits, what is the most it can load and stay at or under 80,000 lbs gross?
According to the AAMVA CDL manual, which three things determine how much liquid to load into a tank?
Product is loaded so that most of the weight is at the rear of the vehicle and the steering axle is lightly loaded. What does the AAMVA manual warn can happen?