I'm going to agree with Lloyd. With all the money the OEM's put into researching fuel milage don't you think they would have tried a $00.13 piece of plasic? You realize performance can also mean fuel milage?
If you are primarily driving longer distances, you might see if an underdrive pulley kit exists for your vehicle. It slows down all the driven appliances reducing parasitic drag. As long as you are driving far enough to recharge the batteries there is usually no problem.
Keeping your foot outta the throttle will increase fuel efficiency the most. Slow down to 60mph max., don't let the rpms climb in each gear. Don't use brakes to slow down just glide off the speed, when it comesd time to buy tires opt for those with low rolling resistance.
An excellant substitute for increased HP is to increase your patience. I drive a semi I have about 420hp on tap... To move 80,000 lbs. I don't have any sympathy for some one sporting 275HP moving an 18,000 loaded truck and trailer let alone the unladen pickup by itself. Honestly just reduce your expectations a bit. You will get there and if it truely is important they will wait.
I know I'm a kill joy. If it's a Ford get in touch with Rousch if it's a Chevy... get in touch with Ford!