Diesel prices have been rising so fast that freight carriers need to update their fuel surcharge rates weekly instead of monthly, or else risk squeezing their profit margins , according to transportation management system (TMS) software provider Magnus Technologies.
That conclusion comes as Austin, Texas-based Magnus reported that the U.S. average retail diesel price rose to $6.29 a gallon in September, up $0.78 from the end of August and 68% above September 2025 levels. Those numbers came from the firm’s “September 2026 Diesel Fuel Snapshot” report.
In fact, September 2026 marked the highest monthly average in U.S. Energy Information Administration (EIA) weekly data dating back to 1994. All four September weeks priced above $5.00, bringing the 2026 total to 25 weeks above that threshold.
The speed of the price increase from August to September matters most for carriers whose surcharge programs reset monthly or are based on prior-period averages. When prices rise this fast, a surcharge priced off last month’s average trails what trucks are actually paying at the pump, and the difference comes out of carrier margin on every load until the surcharge table catches up. At 6.5 miles per gallon, a surcharge that trails the pump by $0.78 a gallon costs a carrier 12 cents a mile, or about $300 per truck on a 2,500-mile week.
“A fuel surcharge is supposed to make fuel neutral, it’s not supposed to be a profit builder for either side or a huge cost burden for the carrier,” said Matt Cartwright, founder and CEO of Magnus Technologies. “September showed how fast that balance breaks when diesel sets a new record hitting above $6.00. If your surcharge resets based off of last month’s price, you’re paying September’s price at the pump while only getting paid on August’s price. The fix is to re-rate against the published index every week, so both sides know what’s reasonable.”
