Am I crazy to expect more than 10 MPG

[MikeB]

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!

 

[Jeff Lewis]

Pay Caesar what is due Caesar.

 

[SirRobyn0]

I'm late to the party but want to put in my two cents.

For your current setup 10MPG is perfectly normal. The 350 TH400 - 4.10 combo is workhorse combo, not a mileage maker.

My truck is a 305 / 700R4 - 3.41 truck. When I drive it to work the commute is mixed country roads with speed limits from 35 - 55mph, about 1 mile on the freeway and then a little city traffic to get to the shop. I get upper 10 to 11MPG on that drive.

On the highway I can get 16, but the truck has to be empty and I have to hang out right at 60MPH, avoiding the secondaries. Realistically my empty highway mileage is more like 14.

My point to all that, unless you drive conservatively, and on the highway, not in traffic, a lot, changing the rear end and transmission will not make much of a difference and you will sacrifice towing and hauling ablity.

Plus you have to think about how much money you'll spend to change the parts, and how much gas you could buy for that money.

In my opinion you'd be better off trying little things in hope of a small improvements.

Choke! You'd be surprised how much of a difference this can make. If you drive the truck cold a lot, or even if you only have a few cold starts per tank of gas it came make a difference. The choke should be set to come off as quickly as possible without causing runablity problems. If the truck loads up when it's cold that's going to hurt mileage. If your thermac and heat riser aren't working, if you get them working you'll be able to set the choke to open more quickly.

Make sure the carb mixture is set to lean best idle. Even though it's the idle mixture, it'll effect the overall mileage.

EGR valve. The main purpose is for emissions obviously, but because it cycles already burned exhaust gas back into the combustion chamber at cruise it replaces some of the mixture and can help with mileage.

Timing. 10degrees is a fine base timing but what about the rest of it? If the vacuum advance or mechanical advance isn't working that will hurt mileage. In all likely hood that stuff is working or your mileage would be lower, but..... If the engine will take a little more timing especially at cruse that could help. Getting an adjustable vacuum canister where you can increase the timing just at light throttle, but not so much that it pings.

I use to have a friend with a relatively heavy foot. He would disconnect his secondaries on his carb for daily driving and he'd reconnect them when he was hauling. He claimed that helped him out with 2mpg.

Again what you are getting is normal for a truck setup like yours, but if you run though some of these checks you might just pick up a little MPG.

 

[peats]

"81 short step, 360(.60 over) 700r4, q jet rv cam hedders, 21/2" duals and 3:90 gears. 10 MPG all day long. LETS GO BRANDON

 

[Rusty Nail]

I have a 1986 K20 with a crate engine with fewer than 5k miles. 350/TH400, 4:10 gears, Edelbrock 1406 carburetor. Truck runs fine, Starts fine. Stops fine. Carb is not that old and I tried a brand new one. All I get is 10 MPG, no matter what. New plugs, new wires. Timing is about 10 degrees BTDC. Any ideas?

Yeah I got an idea...enjoy it while you can and take pix.
Geez.

 

[bucket]

Sounds more like his speedometer is off. I've only ever heard of a 454 getting that kind of mileage with fuel injection and overdrive.

My old '86 C20 454 Burb could knock down 13mpg at 70-75 mpg without any trouble at all. 454, th400, 373 gear and skinny 235/85 tires. The tuning must have been just perfect for highway efficiency. Speedo was correct too.

Oddly, driving around town in the winter produced terrible economy. It often got 8 mpg. My '78 with the same driveline gets 10mpg in the winter, but highway mpg is still 10, lol. Doesn't make much sense, but that's how it is.

 

[80BrownK10]

I get about 16 mpg in my truck. But I am LS-swapped, 3.08 gears, and 33" tires.

Man that makes an LS swap look appealing! I have all that...minus the LS swap!

 

[80BrownK10]

These days you will recover your spent money on fuel economy pretty quick!! Especially if you drive it more often than like every other Saturday afternoon.

 

[tubehead]

WTF ? 350HO crate,performer intake manifold,long tube headers,700r4 3.5 gears in a ford 9 inch,rebulit Quadrajet.
Around town driving like a granny and never out of the primarys I get 16-17mpg and its never getting into OD.
As soon as I drive it hard and into the secondarys it drops to 6-7mpg. Maybe our fuel is better than yours but my bet is the Quady.

 

[Rusty Nail]

Damn that's sharp!

 

[rwljpl]

I have a relatively recent 350 HO crate engine, Holley 670 carb, TH350 3 speed, and 3.73 gearing. 33 inch tires. I can get 11-12 mpg on occasion if I drive the interstate but usually it’s around 10 since I do mostly in town driving. I’m at 2450 rpm at 70 mph. I’d love to replace the transmission with something that has a 4th gear/OD. I just have one fuel tank so I am filling up all the time.

 

[Bextreme04]

WTF ? 350HO crate,performer intake manifold,long tube headers,700r4 3.5 gears in a ford 9 inch,rebulit Quadrajet.
Around town driving like a granny and never out of the primarys I get 16-17mpg and its never getting into OD.
As soon as I drive it hard and into the secondarys it drops to 6-7mpg. Maybe our fuel is better than yours but my bet is the Quady.

It's maybe worth pointing out that you use Imperial Gallons in Australia... which is significantly larger than US gallons(17% larger)

 

[QBuff02]

Man i'm happy when mine gets 5 1/2-6 miles per gallon. And thats at average price of $6/gallon for 100 octane in the tank. But this one isn't built for fuel economy, its measured in smiles per gallon. Lol I think off everything i've read that 12-15 mpg on a well oiled machine is about as good as the average person is going to do with one of these.

 

[JBswth]

I have a 1986 K20 with a crate engine with fewer than 5k miles. 350/TH400, 4:10 gears, Edelbrock 1406 carburetor. Truck runs fine, Starts fine. Stops fine. Carb is not that old and I tried a brand new one. All I get is 10 MPG, no matter what. New plugs, new wires. Timing is about 10 degrees BTDC. Any ideas?

IF you drive at about 50 - 55, you might get 12 MPG, but typically, 10 MPG is about normal for your setup, assuming you drive at 70 - 75.

 

[tubehead]

It's maybe worth pointing out that you use Imperial Gallons in Australia... which is significantly larger than US gallons(17% larger)

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.