I have read many articles that have tested the theory in practice and found no significant differences made by these magnetic fuel devices, despite expected claims made by people who sell them and make the claims.
You could better cut fuel usage and emissions by using proven and tested methods such as a smooth driving style without harsh acceleration or braking, keeping the car properly serviced, running with correctly inflated tyre pressures, having the steering and wheel alignment checked regularly, even down to polishing the car to reduce airflow drag, if it does not work at least it keeps your car looking better.