Gas hog

[75gmck25]

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.

 

[bucket]

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.

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.

 

[SquareRoot]

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

 

[77Dmax]

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

 

[Grit dog]

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.

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.

 

[Shorty81]

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.