Gas Mileage
 

I have a '85 GMC SWB 2WD, 350 engine, Quadrajet, 2.73 axle ratio, TH350, Mild Hydraulic roller camshaft 208/210 at .050 112 LSA. Runs excellent!

I am getting 13-14 MPG.

Is that just the way it is? Was hoping for 15-16

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My bone stock 74 has never done more than 10 mpg even with rebuilt carb and full tune......

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youre doing a lot better than I am ill push about ten mpg myself and its a longbed

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That's a lot better then my truck use to get. Before with a 305 mild cam Ported heads and Holley 600 I was doing good to get 10 on the highway.

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With my 79 350. 4" lift n 35's i was getting 13 highway. I have now straight piped it put on 3" body lift n turning 38's... 12 mpg. But if i get into it (which i do very very often) i get 8... So i can get good milage. But i do plugs rotor cap and wires every year. I've since gotten a ranger so i'm not worried about milage too much anymore, but i still like that i get above 10 lol

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check your driving habits, are you a leadfoot?

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Not a lead foot. I drive 60-65 MPH. Light throttle from start. Engine is healthy with a good tune.

Looks like 13-14 is about average.

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If you did somethings like efi a overdrive and gear the rear to match you could pull that 15mpg maybe a few more.These trucks are big heavy and have the aerodynamics of a brick.

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i have a 355 with flat top pistons and a 284 advertised flat tappet cam. i bet im getting 12 o rmore easy. but the 2.73 gears help

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I have an 84 3/4 ton that started out life as a 6.2 diesel with a 4 speed stick and 4.10 rear end. When I picked it up the po had replaced the diesel with a 400 small block from a rusted out 79 chevy pickup he had. I could get 11.5-12 mpg with it driving 65 down the freeway with the motor screaming. Last fall I put 3.73 gears and a 700 trans in it and the same freeway driving gets me 14 mpg at 75 mph. My 87 burb with a 6.5 banks turbo diesel running the 700 trans and 3.73 gears will get around 19 mpg freeway driving. Also have an 87 blazer with a 350 motor, 700 trans and 4.10 gears with a little taller than stock tires that gets about 14 mpg freeway driving. These are all 4x4s by the way.

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I am thinking of installing a 700R4 and swapping out the 2.73's for 3.42's.

Should be a good match with the 275/60-15 tires.

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the 700r4 with 2.73 will net great mileage. my 165k wore out 305 gets 16-18mpg and I heavy throttle on every start

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I was wondering what the average mpg was on a mid 70's half ton . I've only been tinkering with a 77 C 10 long bed since 2012 but with ,125k miles,350/350 with 3.73 gears , I threw the quadrajet away and installed a 1406 Edelbrock 600cfm. and a week ago after a long 100 mile highway run calculated 16mpg , I did check the plugs and they are a light brown , I live at 6000ft elevation , runs like stink now.

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16 MPG with a 3 speed and 3.73 gears, that is pretty darn good.

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Do you have a partially clogged or old BB style cat. How about headers, a electric fan, or atleast a clutch fan. Is your Q jet adjusted properly.

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I have long tube headers, no cats, 2 1/4 duals, Dynomax Super Hemi Turbos, clutch fan, a fresh Q Jet that performs well.

Initial timing is at 13º total is 36º all in at 2800. GM HEI with DUI coil/cap/rotor. Crane Vacuum advance with Blue/Yellow spring combo and GM weights. Curve starts at 800 RPM. Runs great! just not as good MPG's as I thought I would get with this mild low overlap cam.

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When you say 13-14 average that is not bad for a truck.

As for myself when I shoot for mpg I shoot for hwy, or I state hwy. City mpg is hard to define. Such as if I say I get 20 mpg hwy at 65 mph. That way anyone who tries to replicate my habits can.

What do you get hwy. Or do you always drive on the street for which that would be pretty great even for a new truck.

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This 77 2WD Chev came with the exhaust heat valve wired open and there's no crossover on the dual exhaust setup , I've upped the thermostat from a 160 to a 190 and one step hotter plugs , and a couple of degrees more advance on the timing , seems to run crisper .

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My first new truck in '80, a Big 10 350 auto, got 15.5. This was when the world drove 55 and I could afford to fill dual tanks.

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10, uphill, downhill, loaded, empty, headwind, tailwind, towing, not towing,
76 3/4 350ish, 373 350 turbo

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My 79 K5 blazer with sb400 got 9mpg no matter if you were just driving or pulling a house in low gear.

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i think that alot of these trucks are over geared. my c20 in my sig get 14 city 10 towing and i have got as much as 24 highway(super gas timing way up and doing 65) i avg about 21ish highway at 70. and i am not a nice driver.

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my shortbed 1/2 ton with a aluminum headed 350,th350 and 3.08 running a 275/60/15 got 13 mpg with a sportbike in the back on a 4 hr trip to florida...same truck with a built 454 gets 7 mpg if I take it easy

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I've seen your claim of 21mpg in your sig for a while and will take this as an opportunity to ask you how you do it?

I have an 82 K20 with a 350, SM465 and 3.42 gearing. The average for highway driving is 13mpg and the absolute best was 15 with a strong tailwind. I drive at 55mph and stay out of the secondaries if I can help it. I've noticed that driving anything over 60 makes the mileage drop. The truck is well tuned and runs on ethanol free premium.

My truck being 4wd shouldn't account for a 7 mpg difference. Based on what others have posted I believe my mileage is average for my setup. I know plenty of people who are very pleased to get 21mpg in their brand new fullsize trucks, with better aerodynamics and overdrive.

If there is some secret to getting 20+ mpg from a carb V8 gas powered, non-overdrive truck that possesses the aerodynamics of a brick, please share.

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LOL!!! So true. My 84 C20 crew cab with 350/TH400/4.10 got about 10.5 mpg uphill, downhill, loaded, empty, headwind, tailwind, towing, not towing, backwards or sideways. :haha: