For me it's about keeping a diesel system cleaner , Inc egr , dpf , manifolds , etc . I only get 28 to 32 mpg , tootling about anyways ,so I can't comment on mpg figures , mine are unchanged , plus we're on winter diesel and mpg always drop. I don't think there is a product on the market that will up mpg outright , you may see an increase in mpg from a car that was running poorly before using it ,once things begin to clean up ???? Even then it only states upto 10% on the bottle. So could be as low as 1% impact on mpg figures.
Did you put it in a petrol or diesel , you didn't say .