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I bought an 06' CC, with a 4.8L and a 3.23 ratio. The sticker showed this to be the best combination for MPG... apparently the sticker lied. Sticker showed 16/21 and I have only made it over 15 four times and I have 4500 miles on my truck. On another forum I have been told that a 4.8 w/ a 3.23 on a CC is a bad combination. So my question is, would it help at all if I had the 3.23 traded out for a 3.73, and would it cost more than it was worth? I drive about 500 miles a week, so every mile to the gallon would make roughly $270/yr difference. If I could get another 2 miles over 4 years, that would be $2160.00 (ish).


My set up...



Chevy Silverado

Crew Cab

4.8 w/ 3.23

4500 miles in about 60 days

K&N Air Filter @ about 500 miles

1st oil change @ 4400 miles (time flew)


Driving 60/40 mix of HWY/CTY, almost never rush hour driving.


I've recorded every fill up...



348 - 16.27 (delaer filled up after 20 - 40 test miles)

591 - 14.57

938 - 14.72

1249 - 14.33

1552 - 16.86

1753 - 13.53

2093 - 14.58

2401 - 14.98

2684 - 14.80

3009 - 14.19

3237 - 14.84

3576 - 14.49

3872 - 15.77

4174 - 13.94

4533 - 16.12


TOTAL 4533 (/298.23) = 15.20 MPG

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What you didnt mention was your speed. which has alot to do with it. also what kind of driver are you? do race from stop light to stoplight. or are you a granny driver. something else to look at that is always over looked is tires. proper inflation( which is located on the drivers door tag and not on the tire), correct sized tires, and rotation. all these things can help or hurt fuel mileage and are almost allways over looked.

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Unfortunately, you are a victim of physics.. The EPA sticker is based on a 55 MPH highway speed. I'll assume you drive normal posted speed limits (and not over) which means mileage figures you are getting are dominated by wind resistance. Wind resistance increases exponentially as the speed increases, a complicated way of saying that the difference in wind resistance between 65 and 75 is twice what it is between 55 and 65. You want a gear ratio that keeps your engine RPM low but not low enough that the transmission is always dropping out of overdive at every little hill. I do not believe you will get even .5 MPG change out of changing the gear but you'll probably be a lot happier driving it around town, especially if you every pull anything.


You should think carefully about the K&N, they don't filter as well as paper and load up quicker than paper. Any benefit in air flow over paper is short lived and you potentially let more damaging dirt particles into the engine. BTW, since the air/fuel ratio is computer controlled, any difference in air flow resistance through the filter, while it will potentially limit WOT performacne, will not make any difference in gas mileage (regardless of what the commercials say).


FWIW, your numbers are very close to what I get with my '96 CC 7.3L Diesel. Straight highway at 65 I can get as high as 19, straight highway at 70-75 it drops to 16-17 depending on load and wind. Mixed city is 14-15.. 4spd auto, 4.10 axle (2500 RPM at 70)..

240K miles and still happily clacking along..

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type in that address and you will see there is nothing wrong with your truck,they explain how misleading and that a very,very large # are inflated.I bought me a Chevy HHR Thursday and it says it will get 30 on the hiway,I didnt check it, but the DPS in Lufkin told me it does 95 mph.Kahne,my duramax crewcab dually got better mileage than what you listed

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Guest Soaker



I feel your pain. Every 4-6 months I get a new crew cab and the one you have is just like mine and your number are the same as mine. One of my previous trucks had a 5.3 and a 3.73 gear 4x4 and I would continuiously get 16.2-17.1. This truck ran and towed much better than the 4.8 with the 3.23. I also had 4.8 with 3.42 and the milage was about 15.0.



Maybe you should do what I will do next time. Get a 5.3 with a 3.73 gear. I beleive there is less throttle opening because of a larger engine and more gear therefore better fuel milage.

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I'll keep an eye on it, maybe it will get better as it gets older... hopefully...


BUT, from what I've read, the K&N won't help with efficiency, but it may actually hurt the motor in the long run?


I chose this over a 5.3/3.73 and a Titan (5.8) because I thought the mileage would be better :unsure:

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Well, rather than change my rear end, I decided to pull the K&N based on your advice and that of another forum. I also had to fill up right after the change, so we'll see if the K&N was actually a detriment instead of aid.

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No need for the trailer, just make sure you have money cause I'm bringing my appetite... Better yet, I'll just take you for a ride. I promise you'll love it...




The more the marrier... I'll even turn the tune down, so no one gets their feelings hurt...


In all seriousness, the K & N filter oil tends to clog mass air meters. My dad had a check engine light come on on his Tocoma, after some checking and talking to the guys in service at the dealership he works at. They said, check mass air meter and clean it. Afterwards check engine quit coming in and gas mileage went up...

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