[DiverB]

I can say the throttle response is a little crisper since the rail is getting pure fuel and not fuel/air. The only mileage gain was maybe 1/2 mile better per gallon. I am pretty anal on keeping track of my fuel mileage and fine tuning my tuner to get the best fuel mileage while towing. My truck is set up for towing and efficiency, not speed.

What I did was on an identical trip (same location, road, distance) I hand calculated my fuel consumption with certain settings on my tuner and then made the same trip to compare. I usually see 11.5-12mpg while towing. The one time I drove my truck with no trailer to my parents house (340 mile round trip) I was getting 22.4mpg.

I only use my truck for towing, other than that it sits in the garage until the next time its needed.

You wont hurt your truck by adding the FASS system, but it all honestly you wont see a huge jump in fuel mileage to justify the $635 price tag. I have to have it since my twin CP3 needs the higher volume lift pump, but a stock engine gets by just fine on the factory pump/ fuel filter.

Also you don't need to run "FASS Filters". I run Fleetguard filters that I purchase on line in bulk, much cheaper.