Besides better driving habits,tune up improvements and some types of modifications.The only gadget on the market that might work to improve mpg's without causing problems is called the turbonator or something like that.It's a device that fits under the air filter.What it does is to cause a swirling of the intake air which would help reduce the amount of fuel clumping into larger drops.Of course it would also cause a small drop in airflow cfm's.And it would be most effective in an engine where the air filter is over the intake manifold,as in most carburated designs and throttle body injection desings.Remote air filter and turbo engines probably would gain absolutly no benifits at all.Alot of race car engines have the intake passages in their heads ported in a spiraling pattern to help keep intake air volocity as high as possible,and create turbulance,to help in keeping the fuel as misted as they can.Nascar engines also have an insert directly below the carb,inside the manifold,for the same purpose.They call the insert a turtle.It helps prevent puddling of the fuel under the carb.Improves airflow,valocity,and helps keep the fuel mist suspended in the airflow instead of dropping out as droplets.