IFTA Explained: Why the Cheapest Diesel Might Be Costing You Thousands

If you’re a carrier or run a fleet, you probably try to save money by buying diesel where it’s cheapest. But here’s the catch. The lowest pump price can actually cost you more after taxes.

This is because of IFTA. That stands for International Fuel Tax Agreement. It affects how fuel taxes are tracked and paid across different states. Most drivers look only at pump prices, but IFTA changes the true cost.

Let’s break this down clearly with a real example so you can see how cheap fuel can actually hurt your bottom line.

How IFTA works

When you buy diesel, the price already includes that state’s fuel tax. But just paying that tax at the pump isn’t enough.

At the end of every quarter, IFTA calculates:

• how many miles you drove in each state
• how much fuel you used in each state
• how much tax each state should receive
• how much tax you already paid when fueling

If you paid more than what you owe, you get a refund. If you paid less, you owe the difference.

This is why where you buy fuel matters. Cheap fuel in low-tax states might seem like a deal at the pump, but you could end up losing money when IFTA settles up.

Real example: Illinois vs Oklahoma

• You run a route from Springfield, Illinois to Oklahoma City and back.
• You drive it three times per week for 12 weeks. That adds up to 42,840 miles in a quarter.
• Your truck uses around 7,200 gallons of diesel (at 5.95 MPG).

Now let’s compare two different strategies for buying fuel.

Option 1: buy all fuel in Oklahoma (lowest pump price)

In Oklahoma, diesel is 3.30 per gallon. The fuel tax is 19 cents per gallon.

You buy 7,200 gallons:

• Fuel cost = 7,200 × 3.30 = 23,760 dollars
• Fuel tax paid = 7,200 × 0.19 = 1,368 dollars

IFTA calculation shows what you owe:

• Illinois: 1,113.3 gallons × 0.736 = 819.40 dollars
• Missouri: 4,296.6 gallons × 0.27 = 1,160.08 dollars
• Oklahoma: 1,790.1 gallons × 0.19 = 340.12 dollars

Total IFTA tax owed = 2,319.60 dollars

You already paid 1,368, so you owe:
2,319.60 – 1,368 = 951.60 dollars

Total real cost = 23,760 + 951.60 = 24,711.60 dollars

Option 2: buy all fuel in Illinois (highest pump price)

In Illinois, diesel is 3.73 per gallon. The fuel tax is 73.6 cents per gallon.

You buy 7,200 gallons:

• Fuel cost = 7,200 × 3.73 = 26,856 dollars
• Fuel tax paid = 7,200 × 0.736 = 5,299.20 dollars

Same tax owed: 2,319.60 dollars

You overpaid tax by:
5,299.20 – 2,319.60 = 2,979.60 dollars refund

Total real cost = 26,856 – 2,979.60 = 23,876.40 dollars

What if I think cheap fuel still saves me money?

This is where many carriers get confused. At first glance, it looks like this:

• Oklahoma fuel is 3.30 per gallon
• Illinois fuel is 3.73 per gallon

That’s a difference of 43 cents per gallon

So people assume buying 7,200 gallons in Oklahoma saves:
7,200 × 0.43 = 3,096 dollars

But here’s the truth. That 3,096 dollar “savings” vanishes because of IFTA.
You owe 951.60 dollars in tax if you bought in Oklahoma.
You get a refund of 2,979.60 dollars if you bought in Illinois.

Real difference after IFTA = 24,711.60 – 23,876.40 = 835.20 dollars lost by fueling in Oklahoma

So even though you “saved” at the pump, you still lost over 800 dollars that quarter by choosing the wrong state to fuel in.

The smart fueling strategy

To actually save money, you need to look beyond pump prices.

If you do most of your driving in high-tax states like Illinois, it’s better to buy fuel there too. The higher tax gets credited back to you, which brings your real cost down.

If you always fuel in low-tax states like Oklahoma but drive through high-tax areas, you’ll owe more later. That makes your total fuel cost higher than it looks on the receipt.

Try to buy at least some fuel in high-tax states. Avoid relying only on low-tax zones.

Final word for carriers

Chasing cheap fuel without understanding IFTA is a costly mistake.

The price on the sign doesn’t show the whole story. You could be losing thousands of dollars per year just by fueling in the wrong places.

Fueling smart means knowing how tax credits and refunds work. When you plan right, you don’t just drive more miles – you keep more of your money.

Cheap fuel can be expensive. Know your routes. Know your taxes. Fuel where it pays back.

Ready to stop guessing and start saving? Our premium mileage calculator shows you exactly where your tax dollars are going – and how to fuel smarter.