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Old 06-09-2012, 11:45 AM   #1
Drizzt OP
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What kind of gas mileage does a Chevy 350 realy get?!

In my ponderings for a new 4x4, I keep considering chevy trucks from the 70's or early 80's, whether it is a truck or a blazer or suburban, with a 350 smallblock. However, when researching gas mileage, I get wildy varying reports, anywhere from 6-22.

I'm just curious, what kind of real world gas mileage does a 350 really get, if well tuned, maybe some minor engine mods (free flowing exhaust and intake maybe).

I would also prefer a manual, it looks like they're all mainly 4 speeds, so I don't imagine gas mileage is better then the auto's then. (better durability though). Also, this truck would be a 4x4, so probably some amount of lift (probably no more then 4 inches) and sitting on 33's.

Is it really possible to get more then 16 mpg on a regular basis, if well maintained, or is that just a dream?
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Old 06-09-2012, 12:04 PM   #2
adam_c_eckhardt
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Quote:
Originally Posted by Drizzt View Post

Is it really possible to get more then 16 mpg on a regular basis, if well maintained, or is that just a dream?
Not with a 4" lift and 33" tires.

I had a throttle body fuel injected Suburban with a 350 and it got about 14 mpg no matter what.
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Old 06-09-2012, 12:14 PM   #3
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Had a 350 with throttle body.Extended cab 4x4 with auto 4 sp. Saw 21 mpg ( Canadian gallon ) with a tailwind. That was once and the best I ever got. Heading into a stiff headwind crossing the prairies with 2 DR650,s in the back it was down to 8MPG.

You don't buy a 350 anything looking for decent mileage.
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Old 06-09-2012, 12:30 PM   #4
beachmike1
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Chevy 350 4 x 4 mpg

Okay gas mileage is real simple. It is fuel used per mile. Duh oh but fuel used is in direct proportion to load and part throttle effiency..full throttle is meaningless unless racing.

So here we go. A std short bed carbureted ( it matters) 350 half ton with gearing such that you Turn about 2200 or less rpm in a late 705 to early eights stock truck will be about 16 mpg on the highway at 65 mph.

They were designed at the time to give best mpg at 65 by putting the torque peak not at cruising speed but at cruise in 2 nd gear for autos or 3 gear for 4 speed. Exceptions trailer packages 4x4 and higher payload systems like 3. /4 or one ton truck. These were geared to run closer to the torque peak at 65. This choice is to give good performance for the expected use.

Now why do this and what does this mean to you? First engineers consider the drag on the vehicle. You can determine this easily with a quick set of two way runs on a level road. Find long leve road. Low wind condition. Accelerate to 65 coast to 55 time it. Do this a few times record your data. Do it in both directions both in and out of gear.

Separate test type ( in and out of gear) average the numbers. Weight truck with the crap in it or estimate.

Do the math......power equals work per unit time.

Work equals force x distance
Weight equal mass x acceleration

The air and road friction you determine in the deceleration test.

Say the truck with crap weight 4000 lbs. and you average deceleration rate from65 to 55 is ten sec. You are then decelerating 1.8 feet per sec per sec at 88 feet per sec...I did the math in fps because it is easier....60 mph is 88 fps.

Truck at 88 fps is requiring 4000 / 32 = 125 slugs x 1.8 equals 225 lbs force. 225 / 6.25 equal 36 road horse power at 60 mph

That varies with size drag ( big meats means big drag) ..gearing. Higher numerical ratio mean greater engine drag..lift kit more aero drag.

Say 36road hp... A carbureted engine run rich of peak. At a reasonable rpm under torque peak the thermodynamic efficiency at par throttl will be between o.65and o. 75 pound per hp hour specific fuel consumption.

At 36 hp road..this means about 25 lbs of gas per hour. Auto gas weights about 6.2 lbs. per gallon. This leads to 15 mpg.

Headers fuel injection set to run lean of peak. Tall gearing all improve part thrilled effiecny but not by a lot unless computerized to run the engine at the ragged edge of precognition with high compression ratios and timed port injection.

That is it in a nutshell. Expect crap mile. It's a truck. You want good mileage ride a moped.
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Old 06-09-2012, 12:33 PM   #5
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I get 16 mixed outta my 08 2wd on 33's.

you do the math.
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Old 06-09-2012, 12:57 PM   #6
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Axle gear ratio will play a factor in the fuel mileage too.
Hubs or no hubs on the front axle will affect the mileage, as will the transfer case style - part time or full time 4 wheel drive.

Full time 4WD had no hubs. Part time had either manual hubs or auto hubs.

Transfer case of the 70s up through 1987 trucks:
NP203: full time 4WD (early 70s)
NP205 (70s)
and NP208 - part time.

1988 and later trucks (when the entire truck was redesigned) had IFS (independant front suspension) and used CAD (central axle disconnect) for the front axle engagement of 4WD... in other words, NO HUBS whatsoever.

dr350maniac screwed with this post 06-09-2012 at 01:05 PM
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Old 06-09-2012, 01:18 PM   #7
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The th350 and th400 autos suck gas because of no torque converter lock up. Lock up is good for about 3 to 5 mpg .Th350 did have a lock up option on the last generation...
Lose the clutch fan... 2 to 3 mpg more
lose the 4X4 and get 3 to 5 mpg more.
tune the Qjet for a lean spot at cruise but comes out of it under accel. 1 to 2 mpg.
TBI maybe get you 2 to 3 mpg. not worth a conversion to me..but I like having the get home no matter what of a qjet.
advance the cam on a stock to small profile cam get 3 to 4 mpg
go to the Roller cam retro fit kits (ouch.to the pocket book) 2 to 4 mpg.. but it also take away the oil problem of a flat tappet cam SBC are noted for...and with the shit we get for oil now...
nv4500 behind them is only about 2 mpg over the four gear(actaully three if it an m456 )
MSD only about 1 to 2 mpg..
slight bump in compression and good heads will get you 5 to 10 mpg... yes the older heads were that bad...even cleaned up the will never be as good as the vortec heads.
small tires and correct gears... lift kits and big tires will hurt gas mileage...

so assuming you start out with with 8 to 10 mpg 4x4 of the 1980 vintage.
no clutch fan 2 to 3 mpg
tuning the carb 1 to 2
lock up converter 3 to 5
good head and compression bump 5to 10
good roller cam 2 to 4......
according to my statement the truck should get any where between 21 and 34 mpg...34mpg un realistic..... I agree
but 21 is doable. I have done it on a 1983 half ton 4x4 with a manual over drive four speed.at 130k on the engine...no engine mods and 31 10.5 tires on it. But i did do a fresh timing chain and four degrees advance on the cam....
My full roller 410 small block(410 SBC with a M456 and 4:10 rear gear) in my old truck gets 18+ at (2740 rpm in a lean spot built in to the Qjet NO OD.) 67 miles an hour.. any faster and the aerodynamics of moving a high sitting brick thru the air comes in to play...
17 to 18 mpg with out a topper on it.
18+mpg with a leer fiberglass topper on it. best configuration for mpg on MY truck I have ONCE gotten 19.8 but never really gotten any better then 18.4 ever again I start to get close to the numbers and something happens to kill it.
THe 19.4 happened going to watkins glen in 2001 when gas was still gas.. tail wind only four stops on the one tank
all for toll boths... and never going above 67..
16 mpg with a 900 pound camper in the back.
14 to 16mpg with the topper and open trailer on the back depends on what i am hauling on the open trailer.
12mpg with 28 foot enclosed on the back . And it is taxing the motor a bit on hills. An unlocked torque converter here would help ...with lock up in normal cruise mode.
I just got my hands on a brand new US gear over drive unit. but i am in a toss up of going to a 4l80e nv4500 or the us gear unit.. or just not messing with trying to get an overdrive into a truck that has never really needed it MPG wise.

My .02 cents worth with playing with these old truck for the last 27 years. I have owned a bunch of them some I wish i still had some i was glad to see go...
You can do it but you have to do it in a minimalist type way..
no AC, under driven power steering pump. electric fans and good radiators oil coolers... flame away....
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Old 06-09-2012, 03:38 PM   #8
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Enough with the engineering, it's making my head spin. I've had about 20 of those trucks, every one a 4x4, 350, 4 spd, lockout hubs, 2-3 half tons, the rest were 3/4 ton trucks (half tons are for pussies ) every one of them got 16 mpg if driven conservatively, every one of them got 4" lift kits and 35" mudders, (gumbo, super swampers) after the lifts they all got 10 mpg, except for one, in that one I chucked the 350 for a 454 and actually got 12 mpg. All had carbed engines in fairly stock form, some had headers but most didn't.
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Old 06-09-2012, 04:22 PM   #9
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My 1999 tahoe 2 door 4wd gets 16mpg average. I think that is about as good as a 350 can get. My 2010 traverse...averages 16mpg. That's a fancy v6. If you want good mpg and 4wd truck/suv get a toyota or a jeep. They average 17mpg.
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Old 06-09-2012, 04:41 PM   #10
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Originally Posted by MikeFromMT View Post
16 mpg if driven conservatively, every one of them got 4" lift kits and 35" mudders, (gumbo, super swampers) after the lifts they all got 10 mpg, except for one, in that one I chucked the 350 for a 454 and actually got 12 mpg.

I'm really much more familiar with boat engines when it comes to American V8's, but I have a 91 chevy suburban 5.7 so that's the 350 plant. I've had approximately 100K miles worth of use with this vehicle and it is now in the mid 200's overall in case you want a "long term wear and tear" idea. Ours averages 14 mpg with the use we give it. This is primarily a mix between running to costco for bulk purchasing, hauling persons and luggage 330 miles at a time with no trailer, at speeds between 65 and 80 mph. This is basically a run from Chicago into MI. We rarely use air con. It is almost always loaded fully in the rear compartment with 5-7 people when in use.


But Mike from MT's experience is very similar to what I see in boats, as "mudders" provide more rolling resistance, as do lift kits. Boats are driven at load all the time, there is no "coast" in a boat as the boat is continuously trying to sink into the water and stop. So often we see fewer gallons per hour in boats that have more engine power when used in this scenario, essentially we run at fixed rpm based on the speed you want to go, the pitch of the prop, and the gear ratio of the transmission, so the typical set up is a 350/351 plant with a direct 1:1 gear ratio to the prop, which runs at approx. 3000 rpm to go 30 mph. There is also usually an option for a 1.2-1.5:1 trans with a larger prop with steeper pitch and bigger plant. The equivalent of changing the rear end out and running a bigger engine. These engines often exibit improved fuel consumption under the same loads. Of course they can also consume more when given the chance.
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Old 06-09-2012, 07:42 PM   #11
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If you want decent mileage in one of these trucks (a Chevy, but not with a 350 V-8), go over to the diesel conversions forum at http://www.4btswaps.com

Get a Cummins 4BT diesel out of an old bread truck or similar vehicle. It appears to be a fairly easy swap into almost any full-size 1/2 ton American truck. With the right gearing, these guys claim around 25 MPG in similar trucks.
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Old 06-10-2012, 02:34 AM   #12
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1980s Chevy truck with 350 will mean a 3/4 ton truck.

The 3/4 ton 350/4-speed trucks I've been around returned fuel economies in the teens stock with no special tuning.

As I recall, from 82 to about 86 or 87, the 1/2 ton Chevy/GMC could only be had with a 305.

I had an 83 Silverado with 305, automatic with lockup converter, and 3.42 gearing. It would return 21 on the highway at 55/60 mph (remember the 55 limit?). It was a pure dog, though, when being asked to pull anything. It was rated at something like 155 horsepower!
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Old 06-10-2012, 07:40 AM   #13
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My circa 1995 half ton Chevy pickup got around 16MPG combined with a bone stock 350. Put a 24' camper behind it? 7MPG. Yeah, less than half. And with its relatively puny tank, I actually ran it out of gas as I pulled into a station on the first trip with the camper because it drank it so fast I didn't even notice it until a PASSENGER said "how long can we go on 'E'?" I literally whipped it into the exit I was about to pass and it cut off as I turned into the gas station.

By contrast, a 1997 Suburban with the 454 got 12MPG combined, and then dropped to about 11.5MPG with a decent sized enclosed trailer behind it.

Yes, you can do a bunch of tuning to a 350 to get it into a tolerable fuel mileage range, but you'll have to put 100k miles on it to see your ROI for what all that tuning costs unless you just "tune as you go" as parts where and need replacing anyway. The beauty of the 350 is not in its mileage, but in reliability, availability of parts, and the ability to make it high performance if you wish.


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Old 06-10-2012, 11:36 AM   #14
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It just boils down to physics; weight, wind drag, rolling resistance, mechanical friction in the drive line etc and a defineable amount of energy (btu's) in a gallon of gasoline available to do the work. Gasoline engines are pretty inefficient, 25-30% of the energy does work, the rest is lost as heat and noise. It's that simple a full size truck at highway speeds will get 12-15 average when driven like 99.9% of people do. It doesn't matter much if it's carby or EFI, Dodge, Ford or Chevy, Nisan or Toyota.

My current 92 GMC 454 (G30 van, scaled at 8400#) gets 12-15, the 99 dodge 318 I had prior was 12, the 2001 360 was 12. At work I get a new truck every 18-24 months I'm currently in a 2011 F350 6.2, it gets 12, the 2008 6.0 chev was 12, the 2006 Dodge 5.7 before was 11, the 2005 Ford 5.4 was the worst at 10.

You can squeeze a bit more out of a diesel as they'll let you run leaner ratios, and there's more btu's in a gallon than gasoline and they're generally forced induction. I had a 2003 Cummins HO, it's best ever driving like an old man was just above 20, it scaled at 7700#.

Anyone claiming the mid 20's and up to 30 for diesels from a full size truck is either full of shit or there's details missing on driving style.

Q; Why do you think mileage hasn't changed significantly over the last 25 years in full size trucks?

A; Because the physics of moving a 7000 pound brick at a given speed haven't changed.

Guys will argue they're better now because they have more horsepower, it's irrelevant. The HP will let you get up to speed faster and maintain it under load but that comes with a cost, more horses more hay. At cruise it will take the same amount of energy to move the vehicle down the road regardless of the HP potential of the engine.
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Old 06-10-2012, 03:09 PM   #15
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Originally Posted by Hughlysses View Post
If you want decent mileage in one of these trucks (a Chevy, but not with a 350 V-8), go over to the diesel conversions forum at http://www.4btswaps.com

Get a Cummins 4BT diesel out of an old bread truck or similar vehicle. It appears to be a fairly easy swap into almost any full-size 1/2 ton American truck. With the right gearing, these guys claim around 25 MPG in similar trucks.
Yep or swap a 3-53 detroit, 6.2 diesel or a isuzu 4bd 3.9. If I was going to swap anything into anything it would be a 4bd into a reg cab shortbed 70-80s dodge/ford/chevy full size. In theory you should be able to manage 20-25 mpg. The 6.2 would be pretty easy and cheap into a 70s-80s chevy.

I get around 20mpg in my old beater dodge but I am running a cummins, a small lift, small tires(32) and I drive slow. I actually managed close to 25mpg out of it once but that was going 60mph for 6 hours nonstop. Don't slow down, accelerate, or turn going across texas and good mileage from old big trucks is possible.

To answer your question drizzt- No, I seriously doubt you will get 16 mpg out of a 350 in a 4x4 with a lift. Larger tires, more drag and lots of weight in an old v8 truck is not a recipe for mileage. However if it is cheap enough and does what you need it to do does it really matter?
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