Start with what the official source says
Your base jurisdiction issues your license, sends the return form and publishes the instructions. Those instructions say which vehicles and fuels are covered, how often to file, what records to keep and for how long, and how to round. This guide is a bookkeeping aid, not a replacement. When anything here conflicts with the official instructions, follow the instructions.
Rates also come from the official source. Tax rates per gallon differ by jurisdiction and change over time, and you should take them from the current rate table for the quarter you are filing. The rates in this guide are invented example rates so you can follow the arithmetic.
The three records that feed the return
Trip miles by state. For every trip, record the date, the unit, the starting and ending odometer readings, and the miles driven in each state. Many drivers use electronic logging or a trip-tracking app that reports miles by state. If you use one, keep the export. If you work from paper, a trip sheet with one line per state crossing does the job.
Gallons purchased by state. Record each fill-up by date, state, seller, fuel type, gallons and the unit. Pay attention to where the fuel was bought, not where the card was issued.
Receipts. Keep the receipt or invoice behind every purchase. The instructions list what a receipt must show, and a card statement alone is often not enough. Store a photo the same day, because thermal paper fades. Also check how the instructions treat fuel used for things other than driving, such as a refrigeration unit, so you track it correctly.
How fleet MPG is figured
Fleet MPG is all the miles you drove divided by all the gallons you bought, for the quarter. It is a single average across the fleet, not a per-state number. With one truck it is just that truck's average.
In our example, the truck drove 13,000 miles and bought 2,000 gallons. Fleet MPG is 13,000 divided by 2,000, which is 6.5 miles per gallon. Check the instructions for exactly which miles and gallons count toward the figure.
Net taxable gallons by state
For each state, divide the miles driven there by fleet MPG. That tells you the gallons the truck burned in that state, called taxable gallons. Then subtract the gallons you actually bought in that state. The result is net taxable gallons.
A positive number means you burned more fuel there than you bought there, so you owe tax on the difference. A negative number means you bought more than you burned there, so the tax you paid at the pump covers more than your use, which shows up as a credit on that line. Multiply the net gallons by that state's rate to get the amount for the line.
A worked example with three states and invented rates
Our truck, for one quarter: State A, 6,200 miles and 500 gallons bought. State B, 4,300 miles and 1,000 gallons bought. State C, 2,500 miles and 500 gallons bought. The totals are 13,000 miles and 2,000 gallons. Invented example rates per gallon: State A 0.30, State B 0.25, State C 0.40.
State A: 6,200 divided by 6.5 is 953.85 taxable gallons. Less 500 bought, net taxable is 453.85 gallons. At 0.30, that line is 136.15 owed.
State B: 4,300 divided by 6.5 is 661.54 taxable gallons. Less 1,000 bought, net is negative 338.46. At 0.25, that line is a credit of 84.62.
State C: 2,500 divided by 6.5 is 384.62 taxable gallons. Less 500 bought, net is negative 115.38. At 0.40, that line is a credit of 46.15.
Add the lines: 136.15 minus 84.62 minus 46.15 is 5.38 owed for the quarter. Notice that the net gallons across all states add to zero, because the fleet burned exactly what it bought. Only the rate differences make the dollars move. Your real return will round and combine lines the way the official form says, and it may add surcharges or interest, so treat this as a rehearsal.
How to catch mismatches before you file
Compare total state miles to the odometer. If the odometer says the truck moved 13,150 miles and your state lines add to 13,000, you are 150 miles short, about 1.1 percent. Find the missing trip before filing.
Compare total gallons to your fuel card statements and receipts. Every gallon on the worksheet should have a receipt, and every receipt should be on the worksheet.
Look at the MPG for sense. A heavy truck that suddenly shows a very high or very low average usually means a missing receipt, a duplicate entry or a wrong odometer reading.
See how much one missing receipt matters. Say a 100 gallon fill in State B has no receipt, so you can only claim 900 gallons there. Total gallons fall to 1,900 and fleet MPG rises to about 6.84. Run the same steps and the quarter's result moves from 5.38 owed to about 0.12 owed. That gap came from a lost slip of paper.
Check that each purchase sits in the right state, that receipts are for the right unit, and that nothing from another quarter slipped in.
Make the worksheet a weekly habit
Fill in the worksheet weekly, not on the due date. Reconcile fuel card statements monthly. At quarter end, the totals are ready. A free calculator for the net taxable gallons step is at /bookkeeping/ifta-net-taxable-gallons-calculator, and it uses whatever rates you type in from the official table. Then compare your result to the official form.
Common questions
Where do I find the rate for each state?
Use the current rate table published by the official IFTA source or your base jurisdiction for the quarter you are filing. Rates change, so do not copy an old number or one from an article.
Can I use my fuel card statement instead of receipts?
Check the official instructions. They list what a receipt or invoice must show, and many card statements do not carry every required detail. When in doubt, keep the receipt.
What if I find an error after I file?
Ask your base jurisdiction how to amend a return. Do not guess. A tax professional who knows trucking can also help.
This guide is general information, not tax, legal or accounting advice. Rules, fees and due dates change; check with the IRS, the NJ Division of Taxation or a licensed professional for your situation.