Do you know how much a 50% diesel price increase is costing your business?

And no, you don't need to buy a single gallon of diesel fuel to incur these costs.

A 50% rise in diesel prices can raise what you pay for freight by well over 50%, even if your business never buys a gallon of fuel. The cost reaches you through your suppliers: nearly every product you buy moves by truck at some point, and carriers and distributors pass their fuel costs downstream through surcharges, index-linked freight rates, and contract terms that few buyers review.

When diesel prices climb, most businesses think about their own fleet, if they have one. Far fewer look at the hundreds of supplier invoices that land in accounts payable every month, each carrying a share of a carrier's fuel bill. If diesel rose 50%, would you know how much more your business is paying? For most procurement teams, the answer is no. This guide shows the three places where that cost hides, how to measure it, and what to change in your supplier agreements.

How diesel costs reach your supplier invoices

The increase rarely arrives as a headline price change. It arrives as an accessorial charge: a line item added to the invoice for a service or cost outside the product price itself. On fuel, these lines carry labels such as "fuel," "fuel surcharge," "FSC" (short for fuel surcharge), or "freight adjustment," and they often appear below the product lines.

On a single invoice, the charge is small enough to escape notice. Across a year of purchasing from dozens of suppliers, it can add up to a meaningful share of your total spend. Fuel costs reach your invoices in three ways:

• A separate fuel surcharge, calculated from a schedule in the supplier or carrier agreement.
• A freight rate tied to a published diesel price index.
• Fuel costs built into "delivered" product prices, minimum-order fees, and contract clauses that allow mid-term increases.

1. Fuel surcharges grow faster than diesel

The 50% in our title is a conservative scenario. According to the U.S. Energy Information Administration (EIA), the U.S. average on-highway diesel price reached $6.529 per gallon for the week of September 21, 2026, $2.78 higher than the $3.749 recorded a year earlier. That is an increase of about 74% in twelve months. The EIA also reported that mid-September prices were the highest on record in nominal terms since it began publishing the series in 1994. Prices eased slightly the following week, to $6.382 on September 28, still $2.628 above the year before.

Most carriers and suppliers recover that cost through a fuel surcharge (FSC), a charge that moves with the price of diesel and is billed in addition to the base freight rate.

How a fuel surcharge schedule works

A fuel surcharge schedule has three parts: a base diesel price, a step (the price increment that triggers a change), and a rate per step. Suppliers use two common methods:

• Percentage of freight. The surcharge is a percentage of the freight charge. For example, the schedule adds one percentage point for every $0.10 that diesel is above a $2.00 base.
• Cents per mile. Common on full truckloads. A typical formula divides the difference between the current diesel price and the base price by the truck's fuel economy. With a $2.50 base and 6.5 miles per gallon, diesel at $6.53 produces a surcharge of about $0.62 per mile, or about $310 on a 500-mile load.

Both methods use a fixed base price, and many schedules set that base years ago, when diesel cost far less. Every dollar of diesel above the base then counts toward the surcharge, so the surcharge rises faster than diesel itself.

Worked example: $250,000 in annual freight

Take the percentage schedule above (one percentage point per $0.10 above a $2.00 base) and apply it to $250,000 in annual freight. The 50% row shows the scenario in our title; the last row shows the actual September 2026 price.

In the 50% scenario, the surcharge more than doubles, from $43,750 to $90,500. At the actual September 2026 price, diesel rose about 74% while the surcharge rose about 159%, adding nearly $70,000 a year ($69,500) without a single change to the freight rate. Buyers seldom model this, because the schedule usually appears only in an appendix to the agreement.

2. Freight pricing tied to a diesel price index

Even where no separate surcharge appears, freight is often priced against a diesel index. The most common is the weekly on-highway diesel average that the EIA publishes. The EIA is part of the U.S. Department of Energy (DOE), so contracts often call this series the DOE index. Three details of that index clause decide how much you pay.

Which index and which region

The EIA publishes a national average and separate averages for each region. For the week of September 21, 2026, EIA data put the West Coast at $7.456 per gallon and the Gulf Coast at $6.177, a difference of about $1.28 per gallon.

A supplier that bills against the highest regional index, rather than the national average or the region your goods move through, charges you for fuel it may not be burning. Check which series your agreement names, and whether that series matches your shipping lanes.

How often the price resets

The averaging window matters too. Some agreements reset weekly, others monthly, and others use "the prior period's" price. In a rising market, a fast reset passes increases through at once. In a falling market, a slow reset or a monthly high keeps charges high for weeks after prices ease. Ask for the same reset frequency in both directions.

Fuel costs inside delivered pricing

Freight costs also hide inside product prices. "Delivered" or "freight-included" pricing can carry an embedded fuel adjustment that never appears as its own line. Small-order and minimum-order fees often rise alongside fuel, too. You pay this cost on every invoice, whether or not the word "fuel" appears on it.

3. Contract clauses that let fuel costs rise without notice

Most supply agreements lock in product pricing carefully. The freight and accessorial terms rarely get the same scrutiny, and that is where increases slip through. These are the clauses we see most often, with the protection to ask for in each case:

• Schedules incorporated by reference. The contract points to the supplier's "current accessorial schedule" on its website. The supplier can update that page at any time, and the new rates apply automatically. Ask for: the schedule attached to the contract, with changes only by written amendment.
• No notice requirement. Nothing obliges the supplier to tell you when a surcharge rate, base price, or fee changes. The first sign is a higher invoice. Ask for: 30 to 60 days' written notice before any change takes effect.
• No cap or ceiling. The surcharge has no maximum, so it keeps climbing for as long as diesel does. Ask for: a maximum surcharge rate, or a renegotiation trigger above a set diesel price.
• One-way adjustments. Increases pass through weekly, with no matching obligation to reduce charges when fuel falls. Ask for: symmetric adjustments on the same schedule and reset frequency.
• Surcharge applied to the wrong base. The percentage is calculated on the full invoice, or on freight plus accessorials, rather than on line-haul freight alone (the charge for moving goods from origin to destination). Ask for: a surcharge applied to line-haul freight only, stated in the contract.
• Broad "extraordinary cost" language. Vague clauses that allow price adjustments for "market conditions" or "increased operating costs" can justify new fees mid-term. Ask for: a defined list of adjustable costs, each with documented evidence and your approval.

None of these clauses is unusual, and none requires the supplier to act in bad faith. They favor the party that wrote the contract. Unless someone on your side reviews them, each one works against your budget.

How to measure and reduce your fuel exposure

The EIA's September 2026 Short-Term Energy Outlook forecasts U.S. distillate fuel oil inventories below 100 million barrels in September, and below the five-year low through much of 2027. With supplies that tight, a quick return to lower prices is unlikely, so the steps below are worth taking now.

1. Quantify the exposure

Pull 12 months of invoices from your top suppliers and isolate every freight, fuel, and accessorial line. Then estimate what a further price move would cost: multiply your annual freight spend by the change in the surcharge rate. On the schedule in our example, every $0.10 rise in diesel adds one percentage point, which is $2,500 a year on $250,000 of freight.

2. Ask for the math

Request each supplier's surcharge schedule, base price, index, region, and reset frequency in writing. Then check a sample of invoices against it. An invoice that uses the wrong week, the wrong region, or the wrong base is a billing error you can dispute.

3. Negotiate protections

Seek a written notice period for changes, a surcharge cap, symmetric decreases, and a surcharge applied only to line-haul freight. Where the base price was set years ago, ask to reset it closer to current diesel prices together with the freight rate.

4. Secure audit rights

Build the right to review and recover overbilled charges into every agreement, and use it regularly. An audit clause that no one exercises recovers nothing, so schedule a review of fuel and freight lines at least once a year.

5. Reduce the number of shipments

Consolidating orders and meeting free-freight thresholds cuts surcharges at the source. Fewer, larger deliveries also reduce minimum-order and small-order fees.

For more on how crude oil prices flow through to supplier pricing, see our earlier piece on how rising oil prices drive up delivery costs.

A second set of eyes on your agreements

Most procurement teams know freight costs are rising. What they rarely have is the time to read every supplier's accessorial schedule, reconcile it against a year of invoices, and renegotiate terms while also managing day-to-day demand.

That is where CenterPoint Group can help. Our team reviews supplier contracts and invoice history to identify where fuel surcharges, index terms, and accessorial fees are costing more than they should. We then work with your existing suppliers to correct billing discrepancies and negotiate protections, without disrupting the relationships you rely on.

If you would like to understand what rising diesel prices are costing your business, contact our team for a no-obligation review of your agreements.

Sources

• U.S. Energy Information Administration, What goes into diesel prices?, September 18, 2026
• U.S. Energy Information Administration, Weekly U.S. No. 2 diesel retail prices
• U.S. Energy Information Administration, Gasoline and Diesel Fuel Update
• U.S. Energy Information Administration, Short-Term Energy Outlook, September 2026