In my '05 Xtreme crew cab (same powertrain specs), I was getting 18-19 mpg with 700lbs in the bed and 20"wheels. Modifications included AEM intake, Borla ProXS muffler, Denso Iridium plugs and Supermodulation SuperSparks. I then got a PCM tune from Supermodulation and my fuel economy went up to 19-21 mpg average mixed city/hwy with a best of 24 mpg hwy only with speeds up to 100 mph and averaging around 75-85 mph.

I'm now in a 2004 ZQ8 3.5L 5 speed manual 3.42 rear with 20s and 500lbs in the bed. I'm averaging 22 mpg city/hwy mixed

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Well I'm "box stock" and if I'm doing it correctly only getting 13-15 city

 

To check your gas mileage, fill the tank and set the trip meter to 0. The next time you fill up, see how many gallons it takes to fill your tank. Divide the miles you drove by the number of gallons it took to fill the tank. If you drove 200 miles and you put in 10 gallons, you got 20 miles to the gallon. miles driven/gallons used = miles per gallon.

 

After you determine how mini miles to the gallon (mpg). Then try and figure out how to improve your mpg. I would start with the simplest, easy items first. Clean air filter. Change oil, I would recommend a synthetic oil.(Reduced friction.). How are your driving habits? Jack rabbit starts? Tire pressures? Condition of your tires? Shocks? Let us know what your final mpg are. Good luck.

 

Are you running 10% ethanol, or E85? I have an xtra cab w/ 3.5 and I get 18-19 mixed. I've noticed I get better milage when I fill up at our re-done Murphy's / Walmart. They sell the ethanol free gas now, and I'm thinking they're dumping some of it into the normal blend. Truck seems to bump up to solid 19.

 

Duh....what is a PCM tune???

 

A PCM tune comes from Supermodulation. He can send you a loaner PCM and you send back your PCM to him. He can modify it with computer software to more of a conservative tune for the best fuel economy, 50/50 (mix of fuel economy and more performance), and also a full performance tune. (Least MPG obviously.)


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I have a 06 xtreme standard cab 3.5 auto with the posi 3.73 rear, 49,000 miles and the best I can attain is 16-17 mpg. Bone stock not beat on and well maintained. I don't care much but I just got rid of my 97 2500 ram cummins monster and that got 17 mpg what gives with GM and milage my 4.2 (atlas) trailblazer gets 15 mph

 

20 mpg average over the course of the last 4 years, 2005 Z71 I-5.
I use a free app "Fuelio", that is pretty cool. It calculates cost and a bunch of stuff. I use it on all my vehicles.
For extremes, I get 44 mpg on a motorcycle, and 2 mpg on a two stroke outboard boat!

 

July 2017-June 2019 my avg is around 16.3, with about 2/3 city 1/3 highway, and idling to warm up every day in winter. I log every fill up in a google sheets spreadsheet.
KN cold air intake is my only mod. I'm sure my wrangler duratrac tires don't help my mpg though.

 

I drive an '04 ext. cab auto I-5 with 3.73 gears with 218,000 miles on it and get between 16 - 18 depending on percentage of hwy/city. I'm a little disappointed that it's not a little higher, but I figure I'm driving something that has been very low maintenance and it's paid for. I've been considering a tune from Supermodulation. If I get 16 MPG's and drive 1,500 miles a month, at $2.50/gallon, I spend $235/month on gas. If my mileage increased to 20 MPG's, my monthly gas expense would decrease to $188/month for a savings of $47/month. So, if the cost of the tune is $250 (I have no idea what the cost is) it would pay for itself in less than 6 months. So, I would save $250 the first year and $500/year for the remaining life of the truck. That's not bad. I've seen posts where members ask the price of the Supermodulation tune, but I don't think I've ever seen someone post the actual price. It would be good information to have for me to make that decision. Also, I've seen various reported MPG increases, so I'm not sure that the tune would get me up to 20 MPG's.
I love my truck. I keep the oil changed and keep the tires inflated properly and keep up with all routing maintenance, but -as the name implies - I'm sort of "stuck in my ways" and I'm not going to change my driving habits! I don't dog it like I did my muscle cars years ago. It's not a hot rod - it's a work truck. It's dependable and it's paid for and it more than meets my needs. Yeah, gas mileage is important, but I'm not willing to drive a Prius, Civic or any other little car to achieve that.
Yeah, our MPG's aren't what we'd like them to be, but it's all about deciding what's most important and truly most cost-effective.