Reason for the FASS? Stock lift pump is good for 600+ hp, and you're not there yet. I'd go cheap and reliable instead of hacking in an electric pump on a mechanical motor. Get a gauge on there and find out if you even have an issue there before throwing (expensive) parts at it.

Definitely check fuel pressure. Also, are you compensating for larger than stock tires in your calculations?

hey tate you have a fass on your 3500 do you not like it? i was told that the lift pump on these trucks are weakest link. my plan is 450- 600 hp just tryin to figure mpgs first if this is as good as it gets so be it. also i thought the fass or air dog was just preventative maintance.

no i have not compensated for bigger tire. 285/75r16 stock is 245? how much differance we talkin about? although that was the first thing i did to truck when i bought it..after factory tires were dead. hummmmm.

so question for gear guys what rear end gear would i need to run 285 or 305 and still get factory ratio of 245?

for instance, 245/75r16 compared to 285/75r16 is an increase of 7.19% Someone catch me if I am wrong on this!!

http://www.anycalculator.com/tiresize.htm

Use this in your mpg calculations and see where you are at.

Take your indicated mileage on the odometer, multiply it by 1.0719, divide the result by the gallons put in the tank since the odometer last reset.

From your post above,
309 odometer x 1.0719 ratio = 331.21 adjusted miles / 31.6 gallons = 10.48 mpg.

As always, I look for someone to verify my calculations here....

My 3500 is a 24v, with the computer system still intact. It kicks on when the motor starts, and shuts off once the engine shuts off. It won't run with the key on and the engine off, like it will with a 12v system. The 12v lift pump is not a weak link. They last a long time, and are relatively cheap to replace. My last FASS system was in the $600 range. I bought a spare 12v lift pump for around $150 or so. Theres nothing wrong with the electric lift pumps, but I wouldn't put one on my 12v unless my stock pump was inadequate.

I have been getting 13.8 to 13.4 MPG when towing on the freeway ( 3 horse trailer plus 100 gallons of water) little over 8k, 23~25MPG on the freeway not towing and around 12~14 around town, But I don't have a lead foot, not when Diesel is $4.10