* 22 gallons: This was the standard size.
* 26 gallons: Some models, particularly those with the 8.1L V8 engine, may have had a 26-gallon tank.
To know for sure what size tank *your* specific 2004 Avalanche has, you should:
* Check the owner's manual: This is the most reliable source.
* Look at the fuel filler door: Sometimes the tank size is printed on a sticker there.
* Check the vehicle's sticker: The sticker often located on the driver's side doorjamb will list the GVWR (Gross Vehicle Weight Rating) which might help deduce the fuel tank size. (Larger GVWR generally correlates with larger tank size)
Without knowing the specific trim level and engine of your Avalanche, it's impossible to say definitively.
