IFTA
The International Fuel Tax Agreement, which settles fuel tax across every state you drove through on one quarterly return.
IFTA stands for the International Fuel Tax Agreement. It is the system that splits the fuel tax you owe among the states and provinces you actually drove through, so you file one return instead of dealing with every state on its own.
Here is the idea. You pay fuel tax at the pump wherever you fill up, but you owe tax based on where you burn the fuel. Each quarter you report your miles by state and your total gallons. Your base jurisdiction works out what you owe each state, credits the tax you already paid at the pump, and settles the difference. Some quarters you owe, some you get money back.
The math runs on your fleet MPG: total miles divided by total gallons. Miles in a state, divided by that MPG, is the taxable gallons there, times the state rate is the tax. Indiana, Kentucky, and Virginia add a surcharge with no pump credit. You can estimate the whole thing with the IFTA calculator or check any state on the IFTA rates pages.
Related: IRP, fuel surcharge, rate per mile.