In a program that calculates the average miles per gallon based on miles traveled and gas consumed, the item that holds the miles per gallon should be declared as a variable because it will change depending on the input values. The data type should be a floating-point number (float) because miles per gallon is a value that can have a fractional part, and it is not a fixed value, hence it should not be a constant.

Best practices in programming suggest using variables for values that change and constants for values that remain the same throughout the program execution.

The concept of variables and data types is fundamental in programming and is covered in foundational programming documentation and textbooks.