According to the advertising, Mudrik expects his gas mileage
to increase by
(0.15) x (22) = 3.3 mpg ,
to (22) + (3.3) = 25.3 mpg .
But to answer the question exactly as you asked it,
I would predict that the change in the gas mileage
after installing the device will be insignificant, and
that the change is equally likely to be either an
increase or a decrease.
If such a device existed, it would already be standard
equipment on every car that's sold.