A. There are too many variables to give an exact answer.
These variables include but are not limited to: vehicle make, model, engine size, fuel system, maintenance, condition, fuel saving method, options applied…
In our research, personal experience and customer testimonials so far, mileage gains for simple systems average around 25%. It is possible to double mileage with more advanced options.
Some people notice that their economy continues to increase over time.
Use this calculator to help you estimate the money you can save in 5 years. http://www.eagle-research.com/fuelsav/FiveYrCalc.html