Your fuel surcharge in three numbers, with diesel at $5.454 a gallon

Your fuel surcharge is three numbers and two steps of arithmetic: take the current diesel price, subtract a base price your contract already fixed, and divide by your truck's miles per gallon. What comes out is the extra cents per mile you are paid on top of the line-haul rate.

Start with the current price. Most surcharge schedules are pegged to one figure: the national average retail diesel price the U.S. Energy Information Administration publishes every Monday. In the week to 17 August that average was $5.454 a gallon, up 3.7% on the week and up 46.9% on the year. That EIA number is a retail pump average including taxes, and it is a survey of what stations charge -- fleets buying on contract at truck stops pay less than this, and the gap widens when prices move fast, so read it as direction, not as what you will pay.

That gap is where the surcharge is meant to help you, and it is also where the price gets used wrong. The formula does not take the number on your receipt or your region's price at the pump. It takes the specific figure your contract names, which is almost always the EIA weekly national average for a stated week. Plug in your own receipt and you will not match the settlement; use last week's EIA figure when the contract calls for this week's and you are a few cents off on every mile.

The other two numbers come from your contract, not from any index: the base price and the recovery MPG. The base, or peg, is the diesel price at which the surcharge is zero -- below it you get nothing extra, above it you get paid the difference. The MPG is the fuel economy the surcharge assumes, and a lower assumed MPG pays more per mile for the same fuel price.

Put it together. Say your contract sets the base at $2.50 and assumes 6 miles per gallon. Subtract: $5.454 minus $2.50 is $2.954 a gallon. Divide by 6: about $0.49 a mile. On a 500-mile run that is roughly $246 on top of the line haul. Change only the base to $1.20 and the same math pays about $0.71 a mile -- which is why the base you agreed to matters as much as the price of fuel.

The base and the peg week are the two things to check before you sign or settle. Everything after that is one subtraction and one division you can do at the fuel desk.