Gas hog

My '75 K25 has a 350 with aluminum heads, 213/[email protected] retrofit roller cam, Edelbrock Performer intake, and a Quadrajet that has been tuned to run a little better with the cam. I have stock 31" height tires, 4.10 gears, and my NP203 is still full time 4WD. RPM is about 2800-3000 at 60 MPH.

On a recent 220 mile highway trip I checked the mileage carefully and found I get about 10.6 MPG when driving about 55-65 on the highway. Engine is smooth running and has good power, but it will never get good gas mileage.

Grit dog said:

Winter blend gas usually is only a couple % less efficient. Hardly noticeable. Certainly not grossly worse.
However $120 in 13 days “ not driving much” doesn’t even begin to calculate to fuel mileage and even if it did, no basis for comparison.
Idk if the wet side of the Cascades runs winter gas or not.

All the suggestions above are good places to start.
And it may just be the Eddy carb. I have 2 now one in a big block other on a 350. Haven’t spent the time to try to tune them, but a 750 Eddy on a Gen VI 454 pushing the worlds heaviest 35s with 4.10s = 6-7mpg. And an assuming smaller carb idk what model or cfm on a poochy new 350 1/2 ton with 3.54s is about 11-12mpg.
For reference.
The big one may be running a bit rich, not horribly though. The little one burns clean at the tailpipes.

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The "not driving much" is a useless statistic though. So far, actual travel distance and number of trips has not yet been stated.

My wife says she doesn't drive much either. But it's typically an enormous amount of short trips a few miles to the closest town, many 7 mile trips to the large town, plus one or two 26 mile trips a week to the city. It's mostly the kind of driving that absolutely wrecks mpg.

There's still only two types of men. Those who get 10-12 MPG and those who lie about it. Prove me wrong.

There definitely needs to be quantifiable data before you start throwing parts at it.

75gmck25 said:

My '75 K25 has a 350 with aluminum heads, 213/[email protected] retrofit roller cam, Edelbrock Performer intake, and a Quadrajet that has been tuned to run a little better with the cam. I have stock 31" height tires, 4.10 gears, and my NP203 is still full time 4WD. RPM is about 2800-3000 at 60 MPH.

On a recent 220 mile highway trip I checked the mileage carefully and found I get about 10.6 MPG when driving about 55-65 on the highway. Engine is smooth running and has good power, but it will never get good gas mileage.

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Thats pretty good for what you got.
In the OPs case we have no idea what he’s talking about to even make a suggestion. Sounds like he suddenly is getting worse mileage but unsure of past or current mileage.

86 K20. 496bb, sm465, 208, 31" tires 3:73, 11 mpg. I don't drive over 55. If you want high speed/good mpg buy modern vehicle. Don't try to make something of these trucks something they are not.

I get 19 mpg from April thru Oct 1, then it goes down to 13 mpg.

They water down the fuel and charge you more for it.

3 Ton said:

I get 19 mpg from April thru Oct 1, then it goes down to 13 mpg.

They water down the fuel and charge you more for it.

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Water and gasoline don't mix.

SquareRoot said:

There's still only two types of men. Those who get 10-12 MPG and those who lie about it. Prove me wrong.

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Is that really what you are getting with MPFI and the 5spd?

Frankenchevy said:

Is that really what you are getting with MPFI and the 5spd?

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No, I avg 15 but nobody would believe me so I don't bring it up. I figure my return on investment should take about another 20 years. Lol

I dont think anyones mentioned it
But have you freshened up your ignition system lately?

New plugs, wires, coil etc ?

It's a fcking square body! Lol

Reset your trip counter, or note down the mileage. Drive it, get gas. Divide your miles driven by the gallons of gas you just got and lettuce know that number

^^^^^^^^"Lettuce"??^^^^^^^^.......lol............ Mike?

I'd check the fuel tanks for rusty pin hole leaks FIRST. Then all the fuel lines that go from tank(s) to front. After having a lot of CK's.........their small block mileage is 10 mpg. You can get about 15 mpgs 'tuned right'. Loaded, empty, driven slow, or hammer down. Stick to the Edelbrock performer intake and 600-650 Carb. You can not get a BETTER intake combination for a STOCK small block chevy. Get a strip kit for the carb and throw that Quadrahog as far away as you can. Check your timing. Sometimes the distributor hold down comes a bit loose. You should be at 10-12 BTDC and 35 degres all in. Yeah, some say to run 'em at 8. How about "NO". If it pings, back it down to ten. If you're real good at it.....change out the springs and weights in the distributor. She'll go like a raped ape off the line with that change. Like the other fellas have said,, check all vacuum lines, Make sure your power brake line to the back of the carb is in good. I've seen some dirty vacuum leaks there before. "Cross threaded, not tightened, no teflon etc." 13 days and $120 bucks in fuel. How much is fuel a gallon where you are? Have you or someone else done ANY work on the truck in the last 13 days?