Record Diesel Meets Fall Harvest: Re-Price Your Remaining Fuel Now

By the Thombar team

tl;dr: U.S. diesel set a new weekly record in September, and California diesel now costs $3.20 a gallon more than a year ago. If your season fuel budget was built on last year’s prices, re-price the gallons you still need to buy this week, before the post-harvest bills land.

What’s happening

The U.S. average on-highway diesel price was $6.382 a gallon on September 28, according to the U.S. Energy Information Administration (EIA). That is $2.628 above a year ago, a 70.0% jump. The week before, on September 21, it hit $6.529. That is the highest weekly price in EIA’s records and $0.719 above the previous high of $5.810, set in June 2022 (prices not adjusted for inflation).

The West Coast is paying the most. California averaged $8.181 a gallon, up $3.200 (64.2%) from a year ago, just off its own record of $8.246 set on September 21. EIA’s West Coast region outside California, which includes Washington and Oregon, averaged $6.643, up $2.500 (60.3%).

EIA points to a tight global diesel market and low inventories. In its September outlook, the agency expects U.S. diesel stocks to stay below their five-year low through much of 2027. The same outlook forecasts retail diesel averaging $5.07 for all of 2026 and $4.40 in 2027. So relief is expected next year, but today’s national price is still $1.312 above that full-year forecast.

A note on farm fuel: dyed diesel used off-road carries only a $0.001 federal tax instead of the $0.244 on-road rate, so your delivered price runs below these pump numbers. It still moves with the same market.

What it means for your margin and cash

Fall is fuel-heavy on the West Coast: harvest equipment, hauling to the packer or huller, and post-harvest field work. At retail prices, every 1,000 gallons you still need to buy costs $3,200 more than a year ago in California and $2,500 more in the rest of the West Coast. Your delivered price will differ, so use your own invoices, but the direction is the same.

The timing is what hurts. Fuel bills are due now, while for many crops the settlement check comes later. A fuel line that was fine in the spring budget can quietly eat the cushion you were counting on to carry you into next season’s inputs.

One thing to do this week: re-price your remaining fuel

1. Pull your recent fuel invoices and note your delivered price per gallon.
2. Estimate the gallons you still need through post-harvest work and into winter.
3. Multiply by today’s delivered price and compare it to your budget. The gap is cash you need to find, and a cost you need to cover when you price next season’s contracts.
4. Ask your fuel supplier to quote a fixed price or prepay for your winter volume, and compare it with buying as you go. Prices dipped from September 21 to September 28 and EIA expects lower prices in 2027, so locking in is a judgment call, not a sure thing.
5. If any taxed (undyed) diesel went into farm equipment, ask your CPA about the federal fuel tax credit. The 2025 IRS Form 4136 lists $.243 a gallon for diesel used on a farm for farming purposes, which is $243 per 1,000 gallons.

How Thombar helps

Thombar checks your diesel and gasoline prices every month, tracks your season fuel spend, and alerts you when input costs like diesel, fertilizer and packaging start to threaten your margin.

It can also do the invoice work in step 1 for you: create a custom module, upload your fuel invoices, and Thombar analyzes them automatically and gives you the results.

Sources

• U.S. EIA, Gasoline and Diesel Fuel Update (week of September 28, 2026)
• U.S. EIA, Weekly Retail Gasoline and Diesel Prices
• U.S. EIA, Short-Term Energy Outlook (September 9, 2026)
• U.S. EIA, Weekly U.S. No. 2 Diesel Retail Prices (full history)
• U.S. EIA, Weekly California No. 2 Diesel Retail Prices (full history)
• IRS Publication 510, Excise Taxes
• IRS Form 4136, Credit for Federal Tax Paid on Fuels