Diesel hit a record national average of $6.49 per gallon on September 19, 2026 (Business Today, citing AAA), and FleetOwner reported $6.53 for the week of September 21 amid the Strait of Hormuz disruption. At these prices, the fuel surcharge is one of the largest lines on a carrier invoice, and small errors in how it is calculated turn into real money. A surcharge billed on the wrong week's index, the wrong table or the wrong mileage can look reasonable at a glance and still be wrong by tens or hundreds of dollars per load.

The fix is not to scrutinize every invoice by hand. It is to recompute the surcharge on every invoice automatically, from the terms you actually agreed, and pay only what matches.

Why do fuel surcharge errors get bigger when diesel rises?

Most fuel surcharge programs pay a per-mile amount that rises with the diesel price above a base. The higher diesel goes, the larger the surcharge, and the larger the dollar effect of any mistake in the inputs.

A common structure looks like this:

FSC per mile = (Current diesel index − Base price) ÷ Fuel efficiency (MPG)

Many shippers and brokers use a published table instead of the formula, but the logic is the same. The inputs are the index week, the base price, the MPG divisor or table and the billed miles. Get any of them wrong and the total is wrong.

We covered how to set surcharge programs up in our guide to diesel fuel surcharge automation. This post is about the other side: checking what carriers actually bill.

Where do FSC errors on carrier invoices come from?

FSC errors on carrier invoices usually come from four sources: the wrong index week, the wrong surcharge table, the wrong billed miles, or a surcharge applied to charges it should not cover. Each one is small on a single load and adds up quickly at $6.50 diesel.

• Wrong index week — What happens: Carrier uses the current week's price instead of the week of pickup, or vice versa · Why it's easy to miss: Prices are close week to week, so the error looks plausible
• Wrong table — What happens: Carrier applies its standard table instead of the one in your contract · Why it's easy to miss: The surcharge still looks "normal"
• Wrong miles — What happens: Billed miles exceed the agreed mileage source · Why it's easy to miss: Small mileage differences are hard to spot by eye
• Wrong base — What happens: Surcharge applied to accessorials or a flat-rate load · Why it's easy to miss: Mixed in with other charges

None of these require bad intent. Carriers run many contracts with different terms, and their billing systems default to their own standards. That is why checking each invoice against your terms matters.

What does an FSC error cost at $6.50 diesel?

Here is an illustrative example. Assume a contract with a $1.25 base price, a 6 MPG divisor and an index price of $6.50 for the week of pickup.

Correct FSC per mile: ($6.50 − $1.25) ÷ 6 = $0.875 per mile On an 800-mile load: 800 × $0.875 = $700

Now the same load with common errors:

• Wrong miles (900 instead of 800): 900 × $0.875 = $787.50, an overbill of $87.50
• Wrong table (5 MPG instead of 6): ($5.25 ÷ 5) × 800 = $840, an overbill of $140
• Wrong week (index $6.60 instead of $6.50): ($5.35 ÷ 6) × 800 = $713.33, an overbill of $13.33

The wrong-week error looks trivial on one load. Across hundreds of loads a month, it is not. The wrong-table error, at $140 on a single load, is large enough to matter immediately. These are illustrative figures; your exposure depends on your tables, lanes and volume. The broader pattern of small invoice errors becoming large annual costs is covered in our post on what freight invoice errors cost and how to stop them.

How does an agent check FSC on every invoice?

An agent can do for every invoice what an auditor would do for a sample:

1. Read the invoice in whatever format it arrives, including PDF, email body or portal export, and extract the linehaul, fuel surcharge, miles, pickup date and accessorials.
2. Pull the agreed terms for that carrier and lane: the surcharge table or formula, base price, MPG and mileage source.
3. Look up the right index week, typically the DOE/EIA weekly diesel price for the week your contract specifies.
4. Recompute the surcharge and compare it with the billed amount.
5. Approve matches within a small tolerance and send them on for payment.
6. Flag variances with the specific cause, such as "billed on the September 28 index; contract specifies week of pickup, September 21," and the dollar difference.

The flag is what makes this useful. A rep or AP clerk does not have to redo the math. They see the cause, the amount and the contract term, and decide whether to short-pay, request a corrected invoice or approve.

How do you handle disputes without damaging carrier relationships?

Fuel surcharge disputes are sensitive right now. Carriers are under real cost pressure, and payment delays hurt. A few principles help:

• Pay the undisputed amount on time. Hold only the variance, not the whole invoice.
• Explain the calculation. Send the index week, table and miles you used, not just a lower number.
• Fix the source. If a carrier keeps billing on its own table, update the rate confirmation language and confirm terms with their billing team.
• Answer payment questions fast. Carriers asking about fuel-related short pays deserve a quick, specific answer, a workflow we covered in our post on automating carrier payment inquiries at $6 diesel.

What should you do this week?

1. Confirm which index week your contracts specify and make sure your team applies it consistently.
2. Store each carrier's surcharge terms in a place an agent or auditor can read.
3. Recompute FSC on last month's invoices for your top carriers to size the issue.
4. Set a tolerance, such as a few dollars per load, below which variances are approved automatically.
5. Report variances by carrier monthly so recurring errors get fixed at the source.

Frequently asked questions

How high did diesel prices go in September 2026?

AAA's national average for diesel reached a record $6.49 per gallon on September 19, 2026 (Business Today), and FleetOwner reported $6.53 for the week of September 21.

What is the most common fuel surcharge error on carrier invoices?

Using the wrong index week, the wrong surcharge table or the wrong miles. Each produces a plausible-looking number that differs from the contract.

Which diesel index is used for fuel surcharges?

Most US programs reference the DOE/EIA weekly retail diesel price. Your contract should specify which week applies to each load.

Should brokers hold the whole invoice when the fuel surcharge is wrong?

No. Pay the undisputed amount on time and hold only the variance, with the calculation explained to the carrier.

The bottom line

At $6.50 diesel, the fuel surcharge is too large to check by eye. Recompute FSC on every carrier invoice from the agreed table, index week and miles, approve what matches, and flag variances with the cause and amount, so you pay what you owe and nothing more.

Debales deploys AI agents for carrier invoice audit, fuel surcharge recomputation, variance flagging and carrier payment inquiries. Book a demo.