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Without a doubt a diy 6637 intake, dp, and full exhaust are the best starter options. Then a tuner is an easy mod, just costs a bit more.
 

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In all of the straight drive cars (and diesels) I've owned, I've always noticed increase mileage by riding the rpm's up a bit higher. My 6spd Super Duty got best mileage shifting between 2500-3k rpms. Lower than that and mileage would suffer. Shifting a 5spd at 2krpms will be lugging the next gear out. RPM's dont kill mileage, boost does.
 

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Boost = sea level air pressure x specified boost. If your boosting 25psi, your boosting 25x the static air pressure, thus sucking down much more fuel. I've owned 37cars, most straight drive, and EVERY single one of my straight drive vehicles got better mileage when reving higher.

RPM's do NOT affect diesels because they are extremely lean burning motors. Driving higher rpm's with a very light foot will consume less fuel and utilize less boost.

There is a direct correlation between boost and fuel consumption; boost IS fuel.
 

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I've also found not lugging (that is shifting at a higher RPM say 2500rpm then cruising at 1900rpm) helps fuel econ. Even with my gas truck. Lugging it will knock down the timing a bit and loose fuel econ. Adding some rpm and keeping the timing up helps my econ the most.
Absolutely true! When I had a honda an '05 civic ex as my economy car while at fort bragg, I began testing that theory. My civic was only getting 28mpg while shifting at 2krpms every single time. I dont remember why I started experimenting, but after I started shifting at 2500-3200rpms, while SLOWLY accelerating up there (NOT flooring it to 2500-3200rpms), and after that I NEVER got below 35mpg city. Since then I've experimented with all of my vehicles in this manner, and 9/10 times I get better mileage using a soft foot and shifting at a higher rpm.

I agree with the boost theory.....I try (keyword) to drive mine without raising the boost needle, with the mods in sig and 4.10 gearing, fresh fuel calculation yesterday....17.7 MPG thru 1 full fuel run, in town and hiway mix
Thats exactly what I tend to get, right about 17.7mpg with 50/50 city/highway driving. Which isn't bad because I LOVE to hear my exhaust rip, and because I haul around John Deere's and have the lift/tires.
 
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