DEF (Diesel Exhaust Fluid) is not fuel, but rather, is a part of the exhaust system and is not included in the calculation of your total gallons reported on the quarterly IFTA tax return.
Under the IFTA program, fuel taxes apply to motor fuels. "Motor fuel" means all fuels placed in the fuel supply storage unit of qualified motor vehicles including alternative fuels such as:
- Pure methanol, ethanol, and other alcohols;
- Blends of 85% or more of alcohol with gasoline;
- Natural gas and liquid fuels produced from natural gas;
- Propane;
- Coal-derived liquid fuels;
- Hydrogen; electricity;
- Pure biodiesel (B100);
- Fuels, other than alcohol, derived from biological materials; and
- P-Series fuels; or
- Any other type of fuels or energy used to propel a qualified motor vehicle.