The variable gallonsLeft are not actually being used, it's just to show why one set up is working and the other doesn't. See comment in code. On a calculator I get the same outputs, not sure why I don't in the program. Maybe something to do with data type used? idk...
System.out.println("Tank Capacity: ");
int tankTotal = input.nextInt();
System.out.println("Gauge reading: ");
int userGauge = input.nextInt();
System.out.println("Miles per gallon: ");
int mpg = input.nextInt();
int gallonsLeft = ((userGauge * tankTotal)/100); // This prints out the correct number
int gallonsLeft = ((userGauge/100)* tankTotal); // This prints out 0
System.out.println(gallonsLeft);