Ideas for best gas mileage?

[Farmstore34]

OK I know its a big heavy truck with a thirsty 350, but I need some ideas on increasing the fuel mileage. I currently average 5 to 8 MPG. Heres what I got:
'70 Chevy 3/4 ton 4x4
newer 350 V8
SM465 4 speed
I believe 4.11 gears.
I have a Carter AFB 4 barrell carb, complete MSD ignition system, dual flowmaster exhaust and an open element air cleaner. I am considering:
1. smaller carb (either 2bbl or 500-600cfm 4bbl)?
2. RV camshaft?
3. different exhaust setup?
4. changing out the axle gears (right now I can start out in 3rd without any problem).
5. Taller tires? Currently running LT265/75/16
Any ideas would be great! Thanks!!
Colby.

[bigblockorange]

Swap your rears! Try to find 71-72 with disc's if you can or put a O/D auto trans in it. Oh by the way the O/D trans aren't very strong so don't beet on it!!!!!! Wish you lived closer I could hook you up! Rob

[1fast71]

farmstore where n oregon do u live?

[Farmstore34]

1fast71 I sent you a message regarding your question. Thanks!

[GREASEMONKEY72]

the smaller carb wouldnt really increase the mpg because the cfm is the maxium it goes not what it puts out all the time, taller tires will increase the topend and improve it a bit, the diff gears will improve it the most i think by going to like a 3.07, the exhaust is fine as is, i hope that helped at all

[ND_guy]

Maybe a little tuning or a lighter right foot. I have a 400sbc with HEI, rv cam, Qjet in my k20 with 4.56s and a 4spd. IF I behave my self it gets 9mpg. I know that isnt great but it is 80% better than 5mpg.

With my old 305 I would get 11mpg consistently, but the 400 is so much fun!

[WorkinLonghorn]

What is your freeway RPM? What cam is in there now? What percent Freeway/in-town do you drive? Have you done a compression test lately? Checked the timing curve? I just re-curved mine and am up to 12 MPG which included hauling 2000+ lbs up to Pasadena,and also many full throttle exhibitions of " speed" (not really,it's a heavy truck).

[jimmydean]

I bought a car

I don't even want to know what my K20 gets.

[Yukon Jack]

Hmmm, that gas mileage is about what I would expect for a heavy 4x4 - my 69 K20 with it's 307 averaged about 8-9 mpg before I swapped in the bbc - no idea what its getting now. My 70 K5 with the rebuilt 400 sbc is getting about 9 mpg I think - it also has 4.11 gears and a 4 speed manual. Your setup looks pretty good. I would think it would take a while for a gear change to pay for itself in better mpg.

[Zonaman]

1. Overdrive tranny
2. fuel injection
3. electric cooling fan
4. good tires (see Consumer Reports; Nov2004, Vol. 69 Issue 11, p60-63)
5. add a vacuum gauge to the dash and use it when you drive

[mnunn454]

Most carbs are set from the factory to run a little on the rich side to improve overall driveability and customer satisfaction (and reduce product returns). On most, the jetting can be fine tuned to improve the average air/fuel ratio while highway cruising. If main jetting is already right, then fine tuning the carb's idle circuit can help too.

On modern cars the O2 sensor sends a voltage that's read by the computer which then almost instantly adjusts the fuel injectors and therefore the air/fuel ratio.

Carbs have no such feedback mechanism, but that doesn't mean you can't still benefit from using an 02 sensor. The best air/fuel ratio to try to maintain for comfortable economic driving is 14.7:1 (for the best performance it's 12.5:1 a/f plus or minus a little). 14.7:1 translates to about .5 volts coming off most O2 sensors after they're at operating temperature.

Assuming you don't have other problems (like burnt valves, retarded timing, stuck or bad vacuum advance, vacuum leaks, emergency brakes stuck on, etc) I'd suggest installing a single wire O2 sensor (sensor is about $25 at Auto Zone) in at least one of your exh pipes close to the manifold, and then read its output with an analog voltmeter while you're driving down the highway (keeping one eye on the road of course).

Expect it to fluctuate, but if the meter's needle seems to spend most of its time above 0.5 volts, then the carb is running rich. You can try changing to smaller jetting and see if it improves. Don't jet it to the point it averages below .5 volts because performance will fall off quickly below that level and every time you climb a small hill, it'll feel like you're pulling a semi up with you. I'd shoot for .50 to .55 volts, ideally. (Adjusting the idle circuit is trickier. Let's wait to see what your readings are before spending lots of time discussing this avenue.)

Another thing that might be happening is the back barrels on your carb might be opening partially at highway speeds just to get enough air into the engine at those god-awful rev's you're turning. Try wiring them shut for a tankful or so and see if that helps any. If it does, you may have to go with a carb that has larger primaries just to injest enough air at highway speeds. A vacuum gauge can help monitor manifold vacuum. What's it pulling at highway speeds?

Barring mechanical problems, and once you get the ignition timing and carb mixture dialed in, the only other solutions are taller thinner tires and switching the gears to a 3.07 or 3.42 (expensive conversion for a 4X4, or going with an overdrive tranny (also expensive).

Just my opinion, but I'd think you could see a 10% to 15% improvement by fine tuning the carb and ignition, and maybe 20-30% by swapping axle ratios. I personally don't think you'll ever see much over 13MPG regardless of what you do.

Hope it helps.

[cruiserman]

Park it and ride your bike.

[1970bowtie]

Originally Posted by cruiserman

Park it and ride your bike.

lol...i wish i would have thought of that

[RocketChevy56]

My '69 swb is gettin around 13-14mpg in the city and about 17 or so on the highway. I got a 310ci (305ci .030 over) flat top pistons (compression is around 9.1.1) and a set of IROC heads. Q-Jet and a Edelbrock intake top it off. With a 700R4 and 3.73 posi in the rear, I would suggest just the gear swap first and see where you are. 4.11 are some pretty hefty gears, I would say that drops your mileage pretty bad. What kind of cam do you have? Hope this helps. theres an answer out there somewhere!!



If you give me a couple months, I'll sell you this engine. Its only got 1,700 miles on it. I want to swap this one out, I want a little more power.

[Farmstore34]

Thanks for all the input!
I don't know what cam it currently has, it had a newer crate motor in it when I bought it. It does rumble a bit at idle but is perfectly smooth above that. Plenty of power too. I do most of my driving in it around my farm and around town. Of course I know that short trips with lots of stops/starts is a killer on mileage, but it takes a quarter of a tank to drive to work and back (25 miles total). I think I will try rejetting the carb, its easy to do and I have more than enough power to spare. And as my budget allows I will try the other tips that you'all have suggested. Thanks!
Colby.

[GREASEMONKEY72]

Originally Posted by Farmstore34

Thanks for all the input!
I don't know what cam it currently has, it had a newer crate motor in it when I bought it. It does rumble a bit at idle but is perfectly smooth above that. Plenty of power too. I do most of my driving in it around my farm and around town. Of course I know that short trips with lots of stops/starts is a killer on mileage, but it takes a quarter of a tank to drive to work and back (25 miles total). I think I will try rejetting the carb, its easy to do and I have more than enough power to spare. And as my budget allows I will try the other tips that you'all have suggested. Thanks!
Colby.

rejetting does wonders

[Nytmarek5]

Sometimes Taller gear ratios will hurt MPG best mileage I've ever gotten with my s10 mind you it's a 6cyl. Was 19.4 mpg with a light set of off brand tires towing a trailer from NY to NC second best Was when I swapped tires with my girls jeep and was running 235's (stock it runs BFG 31x10.50 15s) and I got 18.6 consistently until I went back to the larger and heavier style tire and now for the life of me the truck won't get over 16.9 I make the same trip at roughly the same speed every three weeks, so I know my math is good. The smaller tire effetively made my gear set lower and it made my mileage better. Also fourwheelers did a write up years ago where they added a supercharger,4.56 gears 35 inch tires to a wrangler and recorded it's mileage. They actually got 2MPG better with the superchager and lift went from 13 to 15 if I remember right. The article was done to prove performance mods won't kill your mileage if done properly. Biggest issue is mal-tuned carbs.

Just some food for thought take care guys.

[8mpg]

Best option in my opinion is an overdrive transmission and some 3.08ish gears

[cdowns]

synthetic oils in motor tranny and rear as well as grease for the wheel bearings
good alignment ,no 40yo worn suspension pieces . properly balanced and inflated tires as well as tire size matters
well tuned and timed motor
driving style is a biggy. erratic herky jerky will kill your gas tank

[SOON2BLAZE]

well, I have used a gear calculator and came up with 3.50 gears, .73 OD in 4th gear, 28 inch tall tire with 7 inch wide wheel, aluminum drive shaft, hei and an electric fan, no side mirrors, synthetic lube in everything, and all that CDOWNS mentioned. It's still arrow dynamic as a barn door. No idea on milage as it's on jack stands till Friday. I don't think anything else can be done other than sequenchel port EFI to improve it. ( it's A 454) Hot rod magizine wrote that there Crusher Camaro got 26 mpg with a 250 L6 and a powerglide.

[Mertz]

I increase my mileage from 10 to 12 to around 16 by swithing from an Edelbrock to a quadrajet and setting it up with the proper A/F. I also have more power. I have 3.08 gears and 350 tranny with a shift kit and tall 16" tires. That is mostly highway driving with some city. 72 3/4 ton 2 wheel drive.

[dakotajeep]

How much do you really hope to gain from an electric fan? 5-10% ish? That wasnt supposed to sound as sarcastic as it came across!!! I understand it all adds up.


Thad

[1970CustomC10350]

You might try a spreadbore carburator. I like Holley's. The primaries on spreadbores are usually smaller than most two barrels. We run one, I think its a 6210 double pumper on our 3/4 ton and have had pretty good look. I get fuel every two weeks and it has to be about 8 miles one way to work mostly city driving plus some running around. Its not a 4x4 but is a 3/4 ton with RV cammed 400 SBC & sm465. Vacum advance on your disrtibuter will also play a role getting the most mileage out of it as well.

[junkyardjohn]

What the heck is an rv cam? Or for that matter.. A 3/4 cam or a full race cam?? I see them refered to quite often, & i'm curius as to what they actually are. I've installed well over a hundred cams & i've never seen one like that designated on a cam card, or seen any cam manufacturer or cam grinder refer to them in any of their literature. Just silly things like lift, duration & lobe seperation. I've also seen people say "my motor has a cam".. Well every internal combustion motor i've ever had apart had a cam in it. Am i missing something here?? Even the first brigg & stratton motor i rebuilt had a cam. john

[capev86]

taller, smoother tires
3.73/3.42 gears AND a good overdrive (gear vendors ratio only good w/ a 3.07)
fuel injection upgrade to tbi or a well tuned stock q-jet
ditch the 4x4 drive train to save weight and reduce drag on the system
(one of the many reasons i prefer 2wd trucks)

for relief "at the core"......a long stroke/small piston engine......most efficient way to get cubes and make power. there is a reason why the 383 is such a great alternative to the 350...more power and torque without really being more gas hungry.....a longer arm, not a bigger muscle.

or better......a mild tuned diesel transplant with a 3.07 and a GOOD overdrive especially since diesels don't like (or need) to rev high. i have heard of some insane numbers in the 30's for gas mileage out of diesel trucks. couple that with home brewed bio-fuel and you have significant money savings!

my dad got 18mpg on the highway in his '84 burb 1/2 ton 2wd w/ 350, 700-r4, 3.73 gears and q-jet. his present '87 burb has the same exact setup w/ the exception of tbi....same fuel mileage. it also helps to keep the speed down. even super fuel efficient cars get best mileage between 50 and 60mph. as you hit 70 and above the air displacement increases exponentially.....imagine what happens with a big truck with 2-3 times the frontal area going 75mph all day long. and people wonder why they are not getting good gas mileage