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Discussion Starter · #1 ·
i hope you guys can help me im driving a 1985 gmc sierra classic 1/2 ton 4x4 and its getting really bad fuel mileage i havent checked it yet but i dont need to a full tank of gas (15 gal.) will run me for about 150 miles if im lucky or somewhere in there it just seems to drink that stuff like i own the oil well:eek:mg:

truck statistics:

engine: 305 with edelbrock carb & edelbrock performer intake (new air cleaner)

tires: 33X12.5 on 15" rims mud tread wore down quite a bit ( I know thats part of the problem)

so what can you tell me about where i need to start thank you
 

· Retired Aug.31 2007
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I don't want to sound cruel, but if you want better mileage get another vehicle. The one you have equipped the way it is is never going to do very well.
 

· Landscaping Nut
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Tires with less rolling resistance. Keep tire pressure up. A complete tune up if you haven't already.

It would be good to know how well setup carb is, running too rich? Check four wheel drive? How big a drag?
 

· Landscaping Nut
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I don't think you can do much with that. The most fuel efficient gas powered two wheel full size trucks reg cab or extended only get 19 -20. I own a '06 I ordered special with a 3.21 and a 5.3(295hp)l gas that when softly driven only get 19 highway, 17 city/.
 

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Going to be tough to get much better with 33" tires. Sut some skinny stock size tires first.
What gears are in the rear? If they're 4:11's or something close, no matter what you do you'll never get any better unless you go SLOW.
Keep your fooot out of it on the highway. You'd be VERY surprised what 55 will return for mileage.
I tried it once (almost got creamed a few times) with my old PU '87 chevy k10 305 3 spd auto all stock 3:73 gears. I went to NH. Hauled but up there and got terrible mileage _70-75 and she was a screamin'. On return trip babied it at 55 and got almost double mpg! Still nothing to be proud of but it was an old, tired, beat up plow truck.

MU
 

· Gearhead
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Back in my high school days, I put 33's on my truck. It got 8 mpg no matter how I drove it. Didn't need brakes either. Just push in the clutch and those tires would stop it. :biglaugh:

Really with normal size tires, a good tune, and good driving habits, you'd be lucky to get 15 out of it.
 

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Discussion Starter · #7 ·
thanks for the info i knew those tires were going to be a problem I got them off craigslist for 150 bucks but what i really wanted was the aluminum rims that came with them i just got my licence and i really like how my truck looks with them she sits high and proud but im going to get some money and take a big hit and get some all terrains for it, smaller, skinnier, hopefully less rolling resistance.

im running it in full 2wd with the hubs locked out, new spark plugs, carb does need to be re jetted i think with smaller jets, i think it might be a little rich yet but if i move the needles at all either way it runs rougher.

it has a mechanical fuel pump (brand new)

OH and one more question it seems to crank for about 5 secs before it starts when its at running temps after sitting for about 10 mins or over night BUT
if i let it cool, crank it, git her running for a minute then shut it off it will start right back up after sitting what does this mean? thanks for the info
 

· The Magnificent
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First of all, you cannot accurately measure your mileage with those oversize tires. You most likely have not changed the odometer gear to reflect the change in overall circumference. Your true MPG is actually better than the figure you get by dividing indicated miles driven by gallons used.

That being said, I doubt you will see better than 12-14 MPG with this truck.

Consider my Dodge diesel with high pressure street tires gets only 17 MPG average in town and 22 MPG on the highway.

Also as suggested, running 55 MPH max will improve your MPG greatly.
 

· Gearhead
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OH and one more question it seems to crank for about 5 secs before it starts when its at running temps after sitting for about 10 mins or over night BUT
if i let it cool, crank it, git her running for a minute then shut it off it will start right back up after sitting what does this mean? thanks for the info
Sounds like an Edelbrock carb. Everyone I've ever seen has started funny.
 

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That carb (and the Carter it is virtually identical to) is prone leaking off a small amount of fuel when parked. It takes a crank or two for your fuel pump to get it filled back up. The idle mixture screws don't really affect off idle carb jetting, set them where ever the engine idles best. Save up some $$ for gas, hit the interstate for 30 miles at speed, then pull a couple of plugs and see what color they are. In all probability, your carb is going to be pretty close if it still has factory jets.

When I was your age, 55 was a speed for in town, and the sound of my truck pulling away from a traffic light was familiar to many. The only use I had for a tire smaller than 33" was getting to the tire store. (free life lesson- it costs money to move fast and look good, this is a delima that you will face often, trucks are not the only subject matter)


Given the quality of today's gasoline, you are getting typical fuel mileage for what you have. I had 2 K5 Blazers and with 33-12.50's about 11 MPG was normal unless driven very conservatively. You could baby it for 100 miles, then pass one car and you were right back at 11 MPG. The same story with a 440 powered Ramcharger too. The single biggest controllable factor in your fuel mileage results is your right foot. Wish I had a miracle MPG cure, but I don't. Good luck and enjoy the truck anyway.
 

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OH and one more question it seems to crank for about 5 secs before it starts when its at running temps after sitting for about 10 mins or over night BUT
if i let it cool, crank it, git her running for a minute then shut it off it will start right back up after sitting what does this mean? thanks for the info
My dodge does that too. it's because carburators dont seal like FI engines and the fuel will evaporate after a while.

ON the MPG front. Your really not going to see much better mialage. I would sugest going to a skinny 33" if you want to keep the hight. my truck(360, 3spd auto) got 13.5 as the best it ever did back in the mid 90;s when gas was 100% gasoline. it averages 10-12 depending on how hard you accelerate.


. Our '73 IH 1210 2wd long bed crew cab, it's running a 345BB, 2bbl holly carb, full duel exaust with glasspacks, 4spd manual, witout over drive and has 4.55 axle gears riding on stock sized tires, traction on the rear and steering tread on the front. the engine is runngin almost 3 grand(!) at 55, and it gets 11mpg!!!!! Yes your heard me right. it has averaged 11mpg it;s whole life! My dad has papers from trips taken with it and it gets 11mpg empty, 11mpg towing a trailer, and 10mpg with the camper on it. it's amazing.
 

· I'd rather be threshing!
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Like many others have said... lose the big tires, and SLOW DOWN!

I speak from experience! I just took the 305 /75 16's off of my Dodge / Cummins pickup, and went back to the stock 245's. I have not run a tank of fuel out of it yet, but I got 19 on the highway with the 305's. I am expecting low 20's with the stock tires back in place.

BTW... I had never driven the truck with the correct tires on it (bought it with the 305's) and I noticed before I drove 5 miles with it, (after the new tires were put on) how much better the truck handled! I can actually steer it at low speeds again! Much less effort!

I guess my "Cool" factor is gone, but I find the older I get, the "Practical" factor wins out more often!
 

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My dodge does that too. it's because carburators dont seal like FI engines and the fuel will evaporate after a while.
My fuel injected V10 never starts on the first crank. But turns right over on the second try. I noticed when bleeding the fuel system for the valve cover change that there was NO pressure in the line.

I've read that the fuel injectors/seals can start to leak down. Makes me wonder what it's doing to my oil, but I'll change it every few thousand miles.

My 1997 F-250 5.8 was a constant 11 mpg.

And D-Dogg, I'm terribly jealous of that diesel. Except for one trip where I averaged 11 mpg, I've been seeing constant 8 MPG whether loaded/unloaded, highway/city, granny shifting, etc.

I'm not complaining because it does the job - but someday I'll have a diesel!
 

· Dan
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question

are you that light foot? Do you have vacuum gauge? It will tell you if you have heavy foot.


This is why I bought geo tracker to save my money by not use F150 or F250 they get 12-19 mpg VS geo tracker get 30-34 mpg on highway.




Can you go under front axle and try spin driveshaft to make sure 4x4 not still in 4wd that cause bad mpg.


What oringial tire was that before? Then you need change speedometer gear but check with GPS which tell how fast.
 

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I've had a few GM trucks with 305's and 350's with Edelbrock carbs,I got about 13-15 mpg--well tuned carb,fresh tune up,stock 235/15 tires..you could squeak out 17 mph back before we had oxogenated fuels and ethanol in thegas--you lose 3-5 mpg with alcohol in the gas in my experience,especially on older carbed V8 engines..

I'd say ditch the big tires,keep the lightweight rims,put stock tires on it,and drive it gently,if you dont see any improvement I'd have the carb looked at..I used to use either a 500 CFM or a 600 CFM carb,anything bigger is a waste of gas on a small block Chevy..make sure the choke opens fully and quickly--I opted to use a hand choke,because I had flooding and hard hot restarts with the "automatic" ones,they tend to close before the engine needs choking,and wastes gas and fouls plugs..many days above 60 degrees,I didn't use the choke period,or only for the first mile after leaving the yard..when I swapped a 500 CFM Edelbrock with manual choke on my 305 in place of a 600 CFM with an automatic choke,I gained 5 mpg,went from 12 to 17,in a '79 C-10 2wd I had,with 3.73 gears..

I also was able to advance the timing about 4 degrees above stock specs without excessive pinging,that proabaly accounted for some of the mileage increase,also adding a adapter/spacer under the carb to bolt it to a factory quadrajet manifold gave me better driveability and slightly better fuel economy around town..(actually a well tuned Q-jet will beat practically any other 4 BBl carb for fuel mileage,due to the dinky primary barrels--the Edelbrocks are almost all the same size on all 4 barrels,but they are still pretty fuel efficient for a performance carb)...best carb I had for mileage was a Holley "Economaster" 450 CFM 4 BBl..had one on a '74 454 and I got 13-16 mpg with it no matter what--loaded,empty,it never changed!..

Bottom line is your truck weighs 5000 lbs probably,and might be low geared,and have the wind resistance of a plow,so you'll never be getting much over 15 mpg,but you should be able to go 300 miles on a 20 gallon tank I'd think,your right foot has a lot more affect than you think--the vacuum gauge is a good suggestion,I used one in many of my trucks--by watching it you'll notice you can often back off the throttle a lot,yet still maintain the same speed..keep that vaccum higher than 15 inches,and you'll be getting roughly the same amount of mpg..

My 81 G-10 van with a 73 307 V8 and a Holley Economaster 2 bbl carb has gotten 18 mpg on long trips,but its a real dog as far as acceleration--top speed is about 75-80 mph,and its slow getting there--but with 2.73 gears,I can get it up to 72 mph in SECOND!..third just kinda bogs it down,hill climbing is not its favorite thing really..its no fun to drive to VT,NH,or ME,its slow!..but its good on gas!..
 

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Discussion Starter · #16 ·
a lot of good info

ive got the timing set at 12 degrees i set it there because thats where it seemed to idle best

and i know now that looks and speed cost money ha ha lots but i think ill keep my tires till winter im not done showing off yet:bannana: the way ny truck sounds and looks is a real head turner.

course i would assume doing donuts in the gravel parking lot for the city park isnt good for gas either :biglaugh: but i hated the kids that were hanging out there, so i figured their trucks needed a few rock chips:eck16: :sidelaugh

but i do try to baby it when i can it takes a lot to get her off the line

thanks for the advice i really appreciate it :thThumbsU keep it coming
 

· Branded for LIFE !!!
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maybe check if the carb is tight...spray around the base with wd-40..while its running..if idle changes its leaking..i have to retighten mine once a year...B,
 

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I forgot to add to ensure your vacuum advance is working on the distributor (if it has one,some GM trucks used a "EST HEI module with 5 pins that has electronic spark advance with a computer)..if the vacuum advance has a bad diaphram it'll get poor gas mileage and hinder performance (might be why your trucks timing can be set at 12 degrees and idle better?)..also how the vacuum advance is hooked up can affect performance and mileage..if the hose is hooked to a manifiold vacum source,it'll advance the spark most at idle and part throttle,if you put it on a port that only gets vacuum when the throttle is opened (ported vacuum),it'll advance mostly under acceleration..
I found my Edelbrocks poor performers in damp cool weather below 50 degrees,they tended to ice up and cause stalling at idle and horrible gas mileage..one 305 I had one on,I ended up hooking the heat riser (EFE valve) vacuum hoe to manifold vacuum directly so it would stay closed all the time--this forced the exhaust to flow up under the carb through the intake manifolds heat riser passages,and warmed the carb and incoming fuel/air mixture enough so it wouldn't frost up and die at every red light..the adapter/spacer under the carb seemed to increase the icing problems..pre-heating the fuel air mixture is important for vaporizing the fuel efficiently,to get optimum mpg and performance..I only left the EFE valve hooked up like that in fall and winter months,in hot weather it could lead to vapor lock..
 

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Discussion Starter · #19 ·
it does have a vacuum advance set up on it, how do i check to see if its working properly?

what i want to do it convert it to mechanical advance i hear its one of the best performance tricks you can do

my truck seems to run better in damp cool air

ill also check to see if its tight here in a minute
 

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Discussion Starter · #20 ·
ok i just checked the bolts on the carb i got about an 1/8th of a turn on all of them will that make a difference?

also i dont even have the choke hooked up its a manual will it help air flow and performance if i took the choke plate off for now?
 
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