I'm getting 6.88MPG... ideas?
 

Well 3 fill ups ago, I noticed my MPG had dropped from 11-12mpg to 8.5 mpg. My next fill up was 7.33MPG. I cleaned the air filter, and it seemed as though it made a difference, I also decided to change my plugs with NGK TR6s gapped at .035. Afterwards, my car began to stumble/sputter once the car hits 2-2500RPMs. I just filled up my tank, and I got 6.88 MPG on that last fill up. I've started a thread about this on AFB, and many suggested the air filter/plug change etc. But that doesn't seem to have helped any. Also, I changed the fuel pump/fuel filter about 2 months ago...Have any of you experienced this?

Mods are as follows...
RPM stage 3 LS6 Heads
2.02/1.60 valves, been ported on both intake and exhaust side, milled .030
Crane dual springs good to .650 lift
RPM 4 .595/.601 232/236 114lsa.
100 dry shot
LS6 intake
SLP lid and bellows, NGK TR6 plugs, SVO Green Top 42lb injectors, LS1 EDIT done by RPM

. any leaks in lines or tank, how is the exhaust maybe clogged cats if you have them.

Whats your wideband reading while your driving? Are you hitting on all 8 cylinders?

Theres no reason that thing cant get at least 15-20mpg

maybe check fuel pressure/pump and fuel filter

sweets10 is right. u need to do an emission test, basically. those tr6 are a step colder than factory plug, which may not burn hot enough. good luck

thats Freightliner mileage. damn

You should be running more than .035 gap anyway. Might try that

Move your shifter from "1" to "D".

Sorry guys, somebody had to say it!

Clean your MAF. Clean your fuel injectors with a whole can of Sea Foam to 1/4 tank of gas.

Also...One or both of your 02 sensors can very well be failing, not switching properly. You won't necassarily get a code or an SES light until one totally fails either. But they will make you run so rich that you will get horrible gas mileage. You need to get an OBDII scan....from a real OBDII scanner like the dealrships use so you can see how the 02 sensors are switching while the engine is running.

A fuel pressure test with the engine off could tell you if you have a leaky fuel injector under pressure too. That'll kill your gas mileage and cause a stumble at certain rpm's if thats when it starts leaking and dumoing fuel into a cylinder.
If the pressure does fall off you better take the spark plugs out and see which one it is in case you just filled a cylinder with gas. Don't turn it over with the spark plugs in there, you could hydra-lock it.

Also...did you check for puddles of gas leaking under the car anywhere? :)

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Scan readings are??