I have been using this product for about 4 months.
My gas last longer and I can go 300 miles on one tank of gas. My tank is only 10 gallons.
To see how many miles to the gallon you are getting. When you tank is almost empty. Full up your tank, keep the receipt it will show the gallons on it. Now checking your millage, write down the millage on your vehicle or most of you might have a button that you can push to reset your millage.
When you are almost empty again and ready to full up your tank. Get out your last receipt, and then get the millage off your vehicle. You will take millage divided by gallons.