Terrible Gas Mileage

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just_some_guy

just_some_guy

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I'm getting terrible gas mileage. Like really bad. Maybe 6 or 7 MPG.

Motor: 350
Trans: Turbo 350
Gear Ratio: not sure just know it's a 12 bolt rear from a '81 Suburban

Any advice on what could be happening here?
 
AKguy

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Probably need some more information like: Carb equipped? If so, which one? RPMs at 55 mph? Seem to be shifting as it should? Still have the smog equipment (if any from factory)? Tire size?
 
Bextreme04

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I've got a 1980 K25 with 350/TH400/205, 4.10's on 35's, the truck weighs a solid 6,000lbs and even with the motor being hurt and not running right I got better mileage than 7mpg.

that 350/350 combo and the 81 suburban rear end should be doing better than that. Does the motor run good? Nice and smooth with no stumbles or misfires? Does it start right up and idle well even on cold or hot days?
 
Goldie Driver

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Tune up lately ?

Air filter dirty?

Carb need a rebuild ?

Transmission OK ? Is it a TH350C w/ the lockup converter - if so, is that working ?

Lead foot ? :D

Tires aired up ?

Brakes dragging ?


A few things to look at , many of which are really cheap .
 
Last edited:
Dutch Rutter

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My k20 4wd, mild build 350/700r4 (big factor here I know) 2" lift, 33s and 4:10s. Has been consistently getting me 11.8 mpg combined.

My drive to work from the new house is max 55mph and some in town driving. Before it was 90% highway and I'd still get around that 11.8 mpg. I've been feeling pretty good about seeing that out of the old thic girl.
 
CoggedBelt75

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City or highway driving? I have a stroked 350 with an intake swap, TH400, 3500 stall. 4.56 rear + 31” tall tires = 3.82 final ratio. 10 on the highway. In town...I can watch the gauge move. I would think a decent tuneup would bump your numbers up.
 
80BrownK10

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just_some_guy said:
I'm getting terrible gas mileage. Like really bad. Maybe 6 or 7 MPG.

Motor: 350
Trans: Turbo 350
Gear Ratio: not sure just know it's a 12 bolt rear from a '81 Suburban

Any advice on what could be happening here?
Click to expand...
What is this again, without me going back and reading all your posts. That is bad but not far off if you have a lifted k10. I get 10mpg driving normal, and if I stomp it a lot or around town lots of traffic it's closer to 8mpg. I have a k10 sitting on 33s though.
 
80BrownK10

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Dutch Rutter said:
My k20 4wd, mild build 350/700r4 (big factor here I know) 2" lift, 33s and 4:10s. Has been consistently getting me 11.8 mpg combined.

My drive to work from the new house is max 55mph and some in town driving. Before it was 90% highway and I'd still get around that 11.8 mpg. I've been feeling pretty good about seeing that out of the old thic girl.
Click to expand...
You have a 700r4...you have overdrive!!
 
Dutch Rutter

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The OD does make a HUGE difference. It's been some moons now but when I was 16 I had this same engine a little less built in my 86 c20 (was a diesel I swapped) that had a th400 and some very low rear gears. I hotrodded the sh*t out of that truck and probably got 6-8.
 
shiftpro

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Dutch Rutter said:
The OD does make a HUGE difference. It's been some moons now but when I was 16 I had this same engine a little less built in my 86 c20 (was a diesel I swapped) that had a th400 and some very low rear gears. I hotrodded the sh*t out of that truck and probably got 6-8.
Click to expand...

You can kill fuel economy with overdrive as well. If the truck is geared too high it will lug in overdrive and not be operating in the sweet spot. In a heavy truck I like to gear extra low with an overdrive transmission and think of it as a 4th gear rather than overdrive. IMO there is nothing worse than lugging a truck on the highway.
And regarding a 700r4, 'lugging' this transmission makes even more heat and well, they die. Or constantly shifting in and out of overdrive is another sign gears are too high.
 
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