1997 5.7 only 9 mpg

I drive extremely slow and easy, I have downstream o2 codes that shows up a couple months ago but only recently started getting **** mpg

Sounds like there's a trend of low double digit MPG here. My two Vortec trucks are following suit.
'98 Suburban 2500 gets 9 no matter what or where I drive
'00 Yukon Denali daily driver, whether towing or empty gets 9-11 MPG. 8 if I let it warm up in the morning, and plowing all winter.

The way in look at it is this: I didnt buy the truck for MPG. I bought a (then 16 & 170k) 20 year old, 200k mile, red blooded American V8 beast of a motor vehicle for towing my camper, plowing my driveway, and having enough room for my family, groceries, 4 wheel drive traction, and leather seating comfort.

travis mattson said:

I have a 1997 vortec thats getting 9 mpg in town mostly. I was getting 12 mpg. Thinking about doing a ignition tune up and maybe a blackbear tune to help with MPG.
373 gears
4l60e
285 75 r16
New fuel pump
New mpfi injectors
Rebuilt rear diff
Reman trans
New upstream O2s
Downstreams on the way

is there anything I should like certain plugs, tune, tire size etc or is 12mpg the best ill get.

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Friend. Everything appears in order based on your list of maintenance. You have large tires on that beast. I imagine it looks great.
Larger tires have more rolling resistance under acceleration. If your commute is skewed city you're going to see lower than expected mpg vs MSRP window sticker EPA mileage estimates.
You might try even lower (higher numerically) axle ratio if your commute is mostly city. Remember though, it's more to do with your right foot, and an axle swap will take months or years to return your investment via mpg savings.
Still though, you're rolling a gtm400. IMO I'd rather keep it on the road than pay $80k (who am I kidding? I can't pay that, I'd need a loan) for a new one that gets, what 22mpg?
I'll keep trucking at 12mpg city. It's right back to the ROI example above. It'd take me decades (with my commute) to make up the difference in fuel savings.
I like my gmt400. It has AC, power windows, keyless entry, auto trans, and never skips a beat.
$.02

There's two dozen things that can cause poor fuel economy, and not all of them have to do with the engine.

DRAGGING BRAKES Two of my projects last summer was to pull apart the calipers on my K2500, and Trailblazer; both had pistons that weren't moving as freely as they should. Aluminum calipers are especially bad in that regard. The year before, I cleaned-up the calipers on the K1500. This is common as dirt; but it's not the only problem with dragging brakes.
Torque converter clutch failure
Overdrive failure
Plugging catalytic converter--and if you've been getting downstream O2 codes, that's something to look into
Dirty air filter
Low engine cranking compression--worn valves/rings, stuck rings, etc.
Worn/fouled spark plugs
Failed plug wires
Failed charcoal canister/purge valve
Leaking fuel injectors
Excess fuel pressure
Low tire pressure
Lack of lube/failed bearings in differential(s) and/or transfer case
Neighborhood gasoline thief (siphoning)
Fuel leaks under-vehicle
Faulty thermostat--engine doesn't properly warm-up
Failed ignition advance
Faulty knock sensor


The list goes on and on.

travis mattson said:

I have downstream o2 codes

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What do your upstream O2 sensor voltage swings look like? 7 times every ten seconds? Or lazy or no swings?

Downstream O2s are just for diagnostics; they don't actually alter fueling. Upstreams do alter fuel trims.

9 is low for a 1500 unless you're doing a lot of short trips. You might have a leaking fuel pressure regulator in the spider; I've seen that cause crappy economy with no faults if it is just starting to leak. Many things can cause it though, from all the usual tuneup parts to low compression and slipping transmissions. More diagnostic info needed to narrow it down.

My K1500 with 3.42s gets 14-15 city and highway with 31" tall tires. It gets 8 towing a 6x12 enclosed trailer at 75 mph. So your unloaded mileage is what mine is if I'm at 50+% throttle at 3000 rpm in 3rd gear all the time. My Tahoe Limited with 29" tires gets close to 20 highway, 15 city, and 9 mpg towing the same trailer at the same speed. I'm no lead foot, but occasionally use all the power.

gearheadE30 said:

Downstream O2s are just for diagnostics; they don't actually alter fueling. Upstreams do alter fuel trims.

9 is low for a 1500 unless you're doing a lot of short trips. You might have a leaking fuel pressure regulator in the spider; I've seen that cause crappy economy with no faults if it is just starting to leak. Many things can cause it though, from all the usual tuneup parts to low compression and slipping transmissions. More diagnostic info needed to narrow it down.

My K1500 with 3.42s gets 14-15 city and highway with 31" tall tires. It gets 8 towing a 6x12 enclosed trailer at 75 mph. So your unloaded mileage is what mine is if I'm at 50+% throttle at 3000 rpm in 3rd gear all the time. My Tahoe Limited with 29" tires gets close to 20 highway, 15 city, and 9 mpg towing the same trailer at the same speed. I'm no lead foot, but occasionally use all the power.

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My van got about 11 mpg with the 350 in it and 3.73s running 3rd at 3,000 rpm pulling my 23' Jayco at 70 mph. One time I had to pin the throttle to the floor in 2nd gear at about 4,200 rpm, outrunning a storm into a 20-30 mph head wind and sucked down almost half of a 31 gallon tank in 100 miles. Figure I was getting 6-7 mpg. 350 can drink some fuel if leaned into hard. 300 hp marine cam, marine intake and thorley tri-ys at the time.

I’m getting just over 16 hwy. Probably close to what it was getting when new. Look at estimated yearly fuel costs on original sticker.

Caman96 said:

I’m getting just over 16 hwy. Probably close to what it was getting when new. Look at estimated yearly fuel costs on original sticker.

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My 97 Express was rated 12 and 17 and always got pretty close in town and actually better on long trips.

I spent a little time going to cat shows this weekend, all of them out of town, but not far. All less than 1 hour away. Combo MPGs 11.5. most of that was still on town driving.