Fuel math for owner-operators: miles per gallon, diesel prices and fuel tax

Diesel is the one bill that never stops moving. The price changes every week, every state adds its own tax, and your trucks' miles per gallon decide how many gallons you need in the first place. Most owner-operators and small fleets feel all three, but few put numbers on them. This guide does, with worked examples you can check in our free fuel calculator.

The state tax rates below are the IFTA rates for the second quarter of 2026. They change every quarter, so treat them as an example of how the math works, not as today's rates.

1. Your real fuel number is cost per mile

Price per gallon is what you see; cost per mile is what you pay. The formula is simple: price per gallon ÷ miles per gallon = fuel cost per mile.

Example. Diesel at $5.75 a gallon:

• At 6.5 mpg, every mile costs 88.5 cents in fuel.
• At 7.5 mpg, every mile costs 76.7 cents.
• Over 100,000 miles a year, that is $88,462 against $76,667: almost $11,800 a year, one truck, same freight, same price.

That cost per mile is the number to hold every load against. A rate that looks good at 76 cents of fuel a mile can lose money at 88.

2. What moves your miles per gallon

Fuel economy is not luck. A few things decide most of it:

• Speed. Air resistance climbs fast as speed goes up. A common rule of thumb is that every mile an hour over about 55 costs roughly a tenth of a mile per gallon.
• Idling. A heavy truck idling burns somewhere around three-quarters of a gallon to a gallon an hour, going nowhere. Ten hours of idling a week is ten gallons.
• Tires and maintenance. Under-inflated tires, dragging brakes, dirty filters and bad alignment all cost fuel quietly, every mile.
• Aerodynamics and the truck itself. Side skirts, gap reducers and a newer engine add up. An older, paid-off truck that gets poor mileage can cost more a month in extra diesel than a payment on a better one.

Each tenth of a mile per gallon matters. At 2,100 miles a week and $5.75 diesel, going from 6.5 to 6.6 mpg is worth about $1,400 a year, per truck.

3. Diesel prices and the fuel surcharge

When diesel goes up, your rate should follow it through the fuel surcharge (FSC). Many shippers and brokers tie it to the weekly national average diesel price published by the U.S. Energy Information Administration, and a common way to work it out is:

(average diesel price − base price) ÷ mpg basis = surcharge per mile

Example. A shipper uses a $1.25 base price and a 6 mpg basis. With the weekly average at $5.75, the surcharge is (5.75 − 1.25) ÷ 6 = 75 cents a mile.

Two things to know about your own FSC:

• The base price is inside your linehaul rate. The surcharge only covers fuel above it. At $5.75 diesel and 6.5 mpg your fuel is 88.5 cents a mile; the 75-cent surcharge covers most of it, and the rest has to come out of the rate.
• The mpg basis is a gift or a cost. A 6 mpg basis pays you as if the truck needs a gallon every 6 miles. A truck doing 7 keeps the difference; a truck doing 5.5 pays it. Know the basis in every contract you sign.

4. How fuel tax works: IFTA in plain numbers

Every gallon of diesel carries the federal tax (24.4 cents, the same everywhere) and the tax of the state you buy it in. But the truck burns that gallon wherever it drives. If you run in more than one state, the International Fuel Tax Agreement (IFTA) squares it up every quarter: you report each truck's miles by state and the gallons you bought by state, and each state gets the tax on the fuel burned on its roads.

Example. A 2,100-mile week at 6.5 mpg: 900 miles in Texas (20 cents), 600 in Oklahoma (19 cents), 600 in Kansas (26 cents). You buy all 323 gallons in Texas.

• Fuel burned: 138 gallons in Texas, 92 in Oklahoma, 92 in Kansas.
• Tax owed on that fuel: $27.69 to Texas, $17.54 to Oklahoma, $24.00 to Kansas. Total: $69.23.
• Tax you paid at the pump, all in Texas: $64.60.
• On the IFTA return, Texas owes you $36.91 back, and you owe Oklahoma and Kansas their share. Net: you pay $4.63.

Now buy the same 323 gallons in Kansas instead. You pay $83.98 of tax at the pump, and IFTA gives you $14.75 back. The total fuel tax is still $69.23. Where you drive decides your fuel tax; where you buy only decides who holds your money until the quarter is filed.

Two states add a surcharge on top of their IFTA rate (in 2026: Kentucky 10.5 cents, Virginia 14.3 cents). It is owed on the fuel you burn there and is not credited for fuel you buy there, so it follows your miles too.

5. Where to fill up: compare the price without the tax

Because the state tax is settled on your IFTA return by your miles, it is not what one stop saves you over another. What you can actually save on is the fuel itself: the price before state tax.

Example. Two stops on the same run:

• Ohio, $5.69 on the sign, 47 cents state tax: the fuel is $5.220 a gallon.
• Pennsylvania, $5.89 on the sign, 74.1 cents state tax: the fuel is $5.149 a gallon.

The Pennsylvania stop looks 20 cents dearer and sells fuel 7 cents a gallon cheaper. At 323 gallons a week, that is about $1,150 a year for one truck, just from reading the sign the right way.

So a low-tax state is not automatically the cheap place to buy. Compare stops without their state tax, and add what really differs: the chain's discount on your fuel card, and the time it takes to get there.

6. Keep the records that IFTA asks for

An IFTA return is only as good as the records behind it, and an audit asks for all of them:

• Every fuel receipt: date, seller and address, gallons, price, and the truck it went into. A purchase without a receipt is tax you paid and cannot claim back.
• Miles by state for every truck, trip by trip: an individual vehicle mileage record (IVMR) for each trip, which your ELD can produce.
• Four years of it. IFTA records are kept for four years from the return's due date.

Watch your miles per gallon on the return too. A fleet average far from what trucks like yours really get is the kind of number that draws an auditor's questions.

What to take away

1. Know your fuel cost per mile: price ÷ mpg. Hold every rate against it.
2. Every tenth of a mile per gallon is money, on every truck, every week.
3. Know your fuel surcharge's base price and mpg basis.
4. IFTA taxes the fuel you burn where you drive; buying in a low-tax state does not lower your fuel tax.
5. Compare stops on the price without state tax.
6. Keep every receipt and every mile by state, for four years.

Try it with your own numbers

Change the miles, the price and the mpg, put your own states in, and compare two stops in the fuel calculator. Nothing you type is saved.

When you would rather have it done for you every quarter, Mileage Reports reads each truck's miles by state from its ELD, keeps the IVMRs an auditor asks for, and gets your IFTA miles ready. Book a short demo and we will show you on your own trucks.