that wont improve your gas mileage. all it will do is restrict airflow and if anything, cause you to lose power. i've been working with engines for several years now and i've seen people using similar products over and over. it does nothing.
a LESS restrictive intake could yield maybe 5hp or up to 10hp if your stock intake is very restrictive.
i don't see how magnets could improve your gas mileage. if it was that easy, manufacturers would already include this.
overall, don't waste your money on this. if you want to save gas, drive more conservatively and don't go 80 on the highway.