IFTA fuel mileage calculation: the one MPG the return uses, step by step

Step 1: add the miles, taxable or not, in every jurisdiction

Every mile a qualified truck drove in the quarter counts toward the MPG: miles in your base jurisdiction, miles in jurisdictions where you bought no fuel, and miles where the matrix lists no rate (Oregon for diesel in 3Q2026). The miles come from the trip records, which the IFTA Procedures Manual describes with 7 elements (what records to keep for IFTA). Exempt or toll miles are left out of a jurisdiction's taxable miles later; they still count here.

Step 2: add the gallons, every gallon put in the trucks

The denominator is all the fuel placed in those trucks in the quarter: the tax-paid gallons on your receipts, plus bulk fuel and purchases outside IFTA jurisdictions. The IFTA calculator has a "total gallons" field for exactly this case, when the fuel used is more than the receipts show. The receipt matters for a different line of the return, the credit: Credit for tax-paid fuel needs a receipt, invoice, credit card receipt or vendor transaction listing, and only for fuel placed in a qualified motor vehicle with the member jurisdiction's tax in the price.

Step 3: divide, round to two decimals

Fleet MPG = total miles / total gallons, to 2 decimals (P720.350). Sample data, the two trucks the fleet MPG calculator starts with:

Why it is not the average of the trucks

The plain average of the two trucks' MPG in the sample is 6.73; the fleet MPG is 6.58. Averaging gives Truck 102, which drove 12,400 miles, the same weight as Truck 101, which drove 31,200. Only total miles over total gallons makes the taxable gallons on the return add up to the fuel the fleet used, which is what the base jurisdiction checks the receipts against.

What the MPG does next: taxable gallons per jurisdiction

Taxable gallons for each jurisdiction = its taxable miles divided by the average MPG for the fuel type; the MPG is total miles divided by total gallons, rounded to two decimals. Columns for each jurisdiction (miles, taxable miles, taxable volume, tax-paid volume, net taxable volume) with rounding to the nearest whole unit. So a small change in the fleet MPG moves every jurisdiction line; the table shows what 1,000 taxable miles become at each MPG.

Sample data, the quarter the IFTA tax calculator starts with: 8,080 miles on 1,240 gallons is 6.52 MPG, so Oklahoma's 1,420 taxable miles become 218 taxable gallons. The rest of the return, from net taxable gallons to the rate, is in how to calculate IFTA tax.

Miles per state is a different calculation

An "IFTA mileage calculator" usually means miles by jurisdiction: odometer readings at each border, or an ELD or GPS export that records the position and the ECM odometer while the engine runs. That is record-keeping, not arithmetic, and this site does not route trips. Once you have the miles per jurisdiction, the fleet MPG above turns them into taxable gallons in the IFTA tax calculator.