You may want to sequentially record gas-mileage data for at least three fill-ups (or about 1,000 miles driving) before and after installation, respectively, to determine if and how much money the FIR device has actually saved you!
Please be advised that fuel economy is not a fixed number, it varies dramatically based on driver behavior & driving conditions, vehicle condition & maintenance, variations in fuels, inherent variations in vehicles, engine break-in, and so on.
Nevertheless, if you write it down and compare, you should see an average of 10% increase in fuel economy after you have installed the device and let it break in as suggested.
An Alternate Way to Calculate Fuel Economy
If you use your vehicle's trip odometer, MPG can be calculated by
Step 1: Filling the vehicle's gas tank completely and re-setting the trip odometer.
Step 2: When it's time to re-fuel
Filling the tank completely (fill first thing in the morning, if possible)
Writing down the number of gallons it took to fill the tank
Writing down the mileage on the trip odometer
Re-setting the trip odometer
Step 3: Dividing the number of miles driven by the number of gallons it took to fill the tank. The result is the vehicle's MPG for that driving period.
You must write down the data each time for comparing the average over a rather longtime period. Then, you will see an average 10% increase in fuel economy. Otherwise, return the product for refund.