I have always been skeptical of low restriction intake filters helping a gasoline engine with anything other than power at WOT. That's because they don't help "the restriction" from normal driving. "The restriction" is the throttle plate itself. Even going from a 6 month past due dirty stock filter to no filter at all won't change the overall intake restriction one bit.
If you wan't to remove restriction from the intake in a gasoline engine car, remove the throttle first. That's the biggest restriction.
The best ways to reduce intake restriction for fuel mileage reasons are:
- Install a less powerful engine. A less powerful engine will need you to keep the throttle open more, reducing intake restricion.
- Taller gearing. Taller gears means you get less power from RPM and therefore need more toque meaing you'll need to keep the throttle more open.
- Adding EGR or lean burn. The less fuel there is to everything else in the intake charge means it produces less power. Less power means you have to make up for it by opening up the throttle even more.
- Pulse and glide. This gets you better fuel mileage because every time you pulse you are opening up the throttle a whole lot more than you would to keep a steady speed.
- Load your vehicle as much as possible with people or stuff that needs to be moved. This of course helps anyway since putting 5 people in one car is more efficient than having all 5 people drive each his or her own car. The car will need a bit more power to haul more weight, but that helps open up the throttle a little more and reduce intake restriction even more.
- Get a diesel. Diesel engines don't have a throttle. On them you next step would be things like the air filter.
Unless you're driving around at WOT, putting on a lower restriction air filter only means you'll be driving around with your throttle plate closed a bit more.