California diesel just fell 34 cents in two weeks. If your fuel surcharge resets monthly, someone is overpaying.

Here is the mechanism, because it decides who wins. A standard fuel surcharge resets weekly off the federal diesel index. Many shipper and third-party logistics contracts reset monthly or quarterly. When the index moves slowly, the gap between the two is small and nobody notices. When it moves fast, the gap is real money.

It moved fast in both directions this year. National diesel spiked roughly 40% in under two months and peaked near $5.64 a gallon in early April, per federal data. It has fallen every week since, down to about $5.06 nationally and $6.71 in California as of mid-June. West Coast diesel sits at $6.07, still the highest of any region.

On the way up, the lag punished carriers. A surcharge table reset monthly or quarterly kept reimbursing yesterday's lower diesel price while the pump charged today's higher one — the carrier ate the difference. Transportation software firm Magnus Technologies modeled a 55-truck fleet and put the unrecovered cost at roughly $168,000 under a monthly reset and north of $400,000 under a quarterly one during the spike.

Now the move runs backward. A buyer still reimbursing fuel on a table built when diesel was 50 cents higher is paying for gallons the carrier is no longer burning. The lag that cost carriers on the way up pays them on the way down. The structure didn't change — only the direction of the price did.

For an IE operator the question is narrow and worth answering this week: when does your surcharge table reset, and against which index? A weekly reset tracks the market and the gap stays small. A monthly or quarterly reset is a bet on which way diesel is heading — and right now it's heading down. Whoever repriced first, on either side of the freight bill, is the one capturing the spread.

One California wrinkle sharpens it. State diesel runs about $1.65 a gallon above the U.S. average. A surcharge benchmarked to the national index instead of a West Coast one carries a built-in gap that has nothing to do with timing — a separate problem worth a separate look.