Well we just returned from Redmond and got 360 miles on the odometer. We filled up with 20.6 gallons making it roughly 17.5 mpg. I too have the custom cat-back system and attribute most of my mpg gains to this incredible mod.

This was driving with the Power Button on and with us having to go 85mph+ to get around a jerk in a Nissan Altima who sped up everytime we tried to pass him but would then slow back down to 74mph after a failed attempt. We also had a large Sears roof-top cargo box on, two adults and three small children...

I am actually getting worse mpg now that I have the custom chrome intake on the engine...

Brent