Answer: There are a number of ways to know that Ferox Fuel Tabs are working. You could get pre and post treatment emissions tests. Cleaner engines yield fewer harmful emissions. More power and performance is often noticed within the first few treated tanks of fuel. Track your fuel mileage. Improved fuel economy is directly related to a cleaner engine. Check the oil. Due to less carbon blow by, oil will stay cleaner longer. How quickly the Ferox advantage is evident will depend on the history of the vehicle. Some engines will have more deposits than others. Ferox Fuel Tabs are catalytic and will remove hard carbon deposits within 4000 miles of driving. Consistent use of Ferox Fuel Tabs is recommended in order to completely remove all deposits and prohibit their reformation.