How Do You Tell

I have been reading about different products to help you improve your gas mileage but I really am confused on how you can really tell what you are getting. On the highway I get different mileage than on the city streets and on hot days, I definitely get less than cool days. How can I really keep a consistent record and compare it when I try a gas reformulator product? I have difficulty balancing my own checkbook and now I need to figure out calculations for mileage and gas prices. I think I am better off with having someone else figure this stuff out for me.