Am I crazy to expect more than 10 MPG

[SirRobyn0]

Interesting!I didn't know that. That makes more sense of low fuel mileage the poster is getting. I still think the quadrajet is great for economy if you drive gently.

I agree with your thoughts on the Q-jet and mileage if driven gently.

 

[80BrownK10]

Interesting!I didn't know that. That makes more sense of low fuel mileage the poster is getting. I still think the quadrajet is great for economy if you drive gently.

I have a quadrajet and I get regular 10mpg. If I drive like a grand pa and stay out of it and get like 55mpg I will get in the 11's

 

[48hd]

I RUN A 350 ROLLER CAM, 750 EDELBROCK, 350C TRANNY, 2:73 REAR END. I'VE CHECKED IT TWICE.
RUNNING BETWEEN 90-100 MPH FOR 5-6 HOURS, I GET 13.5 MPG. GOT 13.6 THE LAST TIME.
1986 C-10 2WD. WITH THE A/C KICKING.

 

[Matt69olds]

Dads 81 1/2 ton GMC with the 455 Olds swap will average 15 mpg easy on the highway. My dad says he averages 17, but he is also 73 years old, I’m sure he didn’t have nearly the lead foot I do.

The truck still has the 3.08 gears, the TH350 with lockup converter, and most importantly in my opinion, a well tuned Q-Jet and a functional vacuum advance.

Unless your willing to do some tuning on the Edelbrock, and spend the time to dial in the distributor, 10 mpg is about all ou can hope for.

Get the distributor timing curve dialed in. 34-36 degrees of total timing is usually a good starting point.

Get the calibration kit for the carb. Keeping leaning out the primary side until you get a lean miss at steady state highway cruise. That’s the lean limit, then go up a couple jet sizes (or metering rods) then work on the secondary side for best WOT power.

Once v that’s done, work on the vacuum advance.

Read this:

Timing & Vacuum Advance 101 -good article by GM engineer

www.460ford.com

 

[Frankenchevy]

I RUN A 350 ROLLER CAM, 750 EDELBROCK, 350C TRANNY, 2:73 REAR END. I'VE CHECKED IT TWICE.
RUNNING BETWEEN 90-100 MPH FOR 5-6 HOURS, I GET 13.5 MPG. GOT 13.6 THE LAST TIME.
1986 C-10 2WD. WITH THE A/C KICKING.

SOUNDS PLAUSIBLE

 

[80BrownK10]

I RUN A 350 ROLLER CAM, 750 EDELBROCK, 350C TRANNY, 2:73 REAR END. I'VE CHECKED IT TWICE.
RUNNING BETWEEN 90-100 MPH FOR 5-6 HOURS, I GET 13.5 MPG. GOT 13.6 THE LAST TIME.
1986 C-10 2WD. WITH THE A/C KICKING.

Where do you drive at 90mph for 5 hours at a time?

 

[Frankenchevy]

Where do you drive at 90mph for 5 hours at a time?

Pffff… Talladega, duh

 

[Memaloose]

I agree that you're not going to see much better with a 4.10 axle. And you're also dealing with less efficiency of a 3/4 ton axle and TH400. But if 10MPG is your all-around mileage, that's actually not too bad.

If that's a mild 350 and you don't tow more than a small U-Haul trailer, then a 3.08 ratio would work fine and should bump the highway mileage by 2-3 MPG.

An OD trans with lock-up converter would lower highway RPM by at least 30%, but then you'd open a new can of worms. You'd also have to do some cost/benefit math on spending a lot for ring & pinion gears or OD trans plus labor. I mean how long would it take you to recover a $1,000-$3,000 investment?

You might want to play with the calibration on the 1406 carb. I have personally seen 10% better mileage by leaning out a 1406 to the ragged edge. Primary jet size and metering rod diameter are key. Read the theory of operation section to learn how the carb works, and then look at the calibration charts starting on page 5 here: https://www.edelbrock.com/pub/media/wysiwyg/documents/carb-owners-manual.pdf

This just occurred to me: Does the truck have dual exhausts? Properly sized pipes (2-1/4" minimum diameter for a 350) with low restriction mufflers can make a huge difference in performance and gas mileage. I could tell a story about a 30-40% increase in gas mileage after adding duals with generic turbo mufflers, and a 450cfm Holley Economizer 4-bbl to a stock 307, but it would sound like BS!

I had a 1966 Impala SS with a 283 2bbl, 4 speed, headers and a 3:31 differential I think (a long time ago) and I got 20 mpg. It can be done but not with a 350 I don't think. A 307 or 283, yes. Plus, the gearing.

 

[Grit dog]

SOUNDS PLAUSIBLE

You forgot the LOL behind your response!

 

[Grit dog]

Where do you drive at 90mph for 5 hours at a time?

Easy to do out west. My weekly commute is just shy of 320 miles one way. I 90 from one end of the state to the other. Generally 4 hours on the dot door to door with one fuel/piss stop. My cruise control stops at 94 mph, so I don’t cruise faster than that. (Not in a squarebody…)

Drive through Nevada, New Mexico or Eastern Montana and you’ll get bored at 100mph!