If I installed and intake and exhaust on my GS300 would this increase or decrease my MPG?

 

It should increase. Unless you mash the pedal to listen to the sound.. which you probably will...

 

If you were to drive the same as you did before the intake and exhaust install it would increase your mileage, you are adding more air which will make the car run a little leaner which will give more power at any given throttle input resulting in less fuel needed for the same output.

 

I'm running an SRT intake w/ ecu and Tanabe Medallion Touring Exhausts and I can say that the mpg went up to about 2mpg after the intake install and its about the same if not 1 extra mpg after the installation of the exhaust. Should probly total to about 3-4mpg if I dont push the pedal so hard everytime to listen to the sexy growl of the setup =p

 

I put on a HKS Hi-power axle back, Weapon R intake with a K&N Filter, and changed the plugs to Bosch Platinum Ir Fusion..and i went from 17-20 mpg...

 

i dont think it makes a whole lot of difference. 1-2 mpg variance is minimal and not noticeable to me. its all noise to me; i have a drop in + MT on.