What the deal with the gas mileage?

[k72516]

There was a thread back there that asked what you guys were getting for mileage. I don't get how a 305 was getting worse mileage than a 350. It sort of worried me because I was hoping to be getting at least 15MPG all around when I buy my truck. I was thinking of getting a 305 shortbed, or blazer. I thought I would be able to do it with one of those vehicles, but I guess not? I know since I want a carburated engine that's gonna be a challenge, but I figured ya know, it's a shortbed... even with all the extra weight from 4WD... it can be made into a fairly light truck.

Any opinions or thoughts here?

[cliffsta]

The reason a lot of 305s get worse mileage is a lot of them (like my 80) got screwed over with a 2 barrel carb. 350s MOSTLy (but not always) came with 4 barrels, and if you baby the throttle, only the small barrels open up, meanwhile on a 2 barrel, the two barrels are an inbetween size, and are always open, so the MPG suffers a bit.

[ZmOz]

If you're looking for reasonable mileage, you aren't going to find it in a carbureted V8 truck. I'd be happy to get 8 in mine...

[piecesparts]

How about an 84 Shortbed with a 383 stroker, 700R4 tranny, 3.73 posi rear end and 15 MPG to boot. I have gotten this kind of mileage all along and am happy with it. I have two 16 gallon tanks on it and can run approximately 460 miles between fill ups without dragging in on fumes. The trick is to get the combination of gearing and carb size set up to keep the motor in it's power range and not over rev it. I run about 2700 RPM at 75 MPH and it is just under the secondaries on the carb. I can jump it to a 100 MPH with very little effort with the secondaries. I have run this truck on more than one Hot Rod Power Tour trips and the mileage is as good as any and it is repeatable. I have a fairly healthy Comp Cams roller cam with a 2400 stall convertor and then I run a Carter 625 CFM carb for optimum air flow to the engine. The motor could handle a bigger carb, but it would kill off my daily driving and only give me more of a top end on RPMs. I ran a 750 carb for awhile and it caused me to have a lot of bogging and slobbering around the stoplights when driving on the street. My son's truck has a 350 engine with the same carb, tranny and rear end setup, it gets about 14 to 14.5 MPG average. This seems to work well and the cruise controls work easily on these trucks. His truck had a 3.08 rear end with the 700 R4 and it got 11 MPG and it ran pretty lousey in the hills in our area. The cruise was always kicking down to get the vehicle up the hills. There went the gas mileage. Oh; by the way, if you drive it like you stole it----the mileage will always be bad. Set it up to drive good and then run it hard when it needs to be run you can do reasonably well.

[Tom]

Stock 305's and 350's aren't very efficant, so they get bad milage. I got 15mpg freeway with a vortec 350 from a 99 pickup, overdrive and a 3.07 rear while lugging the engine. If I had gone to 3.73's I would have picked up another 2-3mpg. Using a LS1 based engine would have gotten even better milage.

As for the whole 2bbl vs 4bbl in milage thing, its nonsense. The throttle blades only open enough to give you the XXX amount of air flow [cfm] that your asking for to hold your speed. If you need 200cfm of air to keep 60mph, it dosen't matter if its 1/2 throttle on a 2bbl or 1/4 throttle on the 4bbl, your still only adding fuel to 200cfm of air.

[k72516]

LS1 motor... I've always wanted to do that but it'd take me about seventy years to do all the wiring and computer mumbo jumbo. lol

[cliffsta]

Originally Posted by Tom

As for the whole 2bbl vs 4bbl in milage thing, its nonsense. The throttle blades only open enough to give you the XXX amount of air flow [cfm] that your asking for to hold your speed. If you need 200cfm of air to keep 60mph, it dosen't matter if its 1/2 throttle on a 2bbl or 1/4 throttle on the 4bbl, your still only adding fuel to 200cfm of air.

If you say so That's just what I've always been told and heard on here. A properly set up Quadrajet is gonna beat a DualJet in MPG.

[bowtier]

I just checked mine last week. 400 s.b., & 3.73's w/ a 750 edelbrock, in the '71 c10... I was always half scared to check it so I put it off, now I consider it my economy vehicle @ 12 mpg!!! I was so happy I almost pi$$ed myself! ;-) Still haven't got the hair to check my '86 I know it'll be worse.

[sameyrasmea72]

I just made a 692 miles one way trip to Kansas City in my 80 2wd 305 blazer and got 18/mpg with the ac off and 16 on the way back with ac on.
I was going about 60-65mph though. If I bump it up to 70 it gets about 14 usually.

[Brad_Olson]

Originally Posted by Tom

As for the whole 2bbl vs 4bbl in milage thing, its nonsense. The throttle blades only open enough to give you the XXX amount of air flow [cfm] that your asking for to hold your speed. If you need 200cfm of air to keep 60mph, it dosen't matter if its 1/2 throttle on a 2bbl or 1/4 throttle on the 4bbl, your still only adding fuel to 200cfm of air.

Intake charge velocity will vary with the size of the carb venturi and that in turn affects fuel atomization and cylinder filling, which can affect engine performance and efficiency. I'll bet that if I replaced my Q-Jet with a Holley Dominator and drove exactly the same way I'd see a noticeable drop in MPG...

Brad

[86swb]

Originally Posted by sameyrasmea72

I just made a 692 miles one way trip to Kansas City in my 80 2wd 305 blazer and got 18/mpg with the ac off and 16 on the way back with ac on.
I was going about 60-65mph though. If I bump it up to 70 it gets about 14 usually.

Then I would have to be getting 14mpg. 60-65? I was doing that when Nixon gave us the 55 mph on the interstate Hwy.

[Earl Filter]

When I get my newly-acquired truck up and going, I can't wait to see my gas mileage...it will be in gallons-per-mile, not miles-per-gallon. See my signature...

[ocbaud]

the best i could get in my truck, with highway and city driving, with me taking it VERY easy on the truck, was about 11mpg

[kevinr1970]

Here goes:
'86 GMC 1/2t 4x4 - 305, 700r4, 3.73's, 31x10.50's = 12 - 12.5 mpg
'82 K5 2wd - 350, T350, ? gears, 255 60 15 = 12.8 - 13.2 mpg
This is mostly freeway driving back and forth to work.
I have a line on a '89 1/2t 2wd GMC Suburban for about $500. I am thinking that the TBI 350 and 700r4 transplanted into my 2wd K5 would give me a little jump in mileage. And a Suburban front clip, 5 - 15x8 rallyes and a ton of other parts to boot!
Oh, and I wouldn't do the transplant just for mileage, but for better all around driving.

[mysticdragon]

hmmm.... not good news on the MPG ... was actually looking for a thread that mentioned this.

I got an 85 C20 3/4 ton and I get about 8-12 MPG depending on driving conditions. Is this a good average for this truck? What can I do to increase the mileage that is cheap???

ttfn
Mystic.

[72MARIO]

Thats about what I get in my 84 C20. It sucks. I get better economy with 94 gas.

[Ogier]

ok, here goes. I know this is going to get a lot of poopoo-ing but I assure you it is true as I am the one who put it together.

1975 Chev 1/2ton stepside.

350HP 350 SB. Ballanced and blueprinted. 2000rpm stall converter and a stage I shift kit in the tranny. Port, polish and CC'd the heads and decked the block. 10.5 to 1 compression.

Farmer john skinny-by-tall Bald Eagle Readyblow tires..

Out ran a turbo targa porsche on the freeway to 110mph.

Got 24 MPG on the hwy and about 18 in town..

Fuel economy is not hard when you build a system around the vehicle.

Incidently, the motor had a 600cfm holley and hedman headers and a stock intake manifold.

[1FaastC10]

tired old 350 in my '77 backed by a TH350, and a full time NP203, 4.10 gears, and 31" tires gets 12 mpg with the blade on the front... milage is all in the tuning and driving style.

[bumpsburb]

My 87 suburban gets 10 MPG in the city and about 14 on the higheay. I do drive 70 to 80 on the highway. In December I put in a crate motor and had the transmision rebuilt. Put 10 x 30 tires on it. Not sure of the gearing in the rear end but what type of MPG increase would i expect if lowered the gears.

Bump

[1FaastC10]

depends on what you have now really...

[piecesparts]

This is a little like going to the doctor; you haven't told us everything. What do you have for a carb, what RPMs do you run at those driving speeds, and what gears do you have? (you can find out) When you ask about changing to lower gears, are you wanting to get better traction or are you looking to get the motor RPMs back to a good level? Lower will give you more pull on those tall tires but will raise your RPMs on the highway, this might be good if you are lugging the motor all of the time. I imagine that you may be running in the 1700 to 2000 RPM range at 70 MPH wh en you are out on the road and that is probably making you work the engine harder in any inclines or hills. The suburban is a heavy vehicle and 14 MPG on the highway is good in all aspects. Think of pushing a very large brick down the road at 70 MPH and what energy would be necessary to accomplish that. Add to that tall tires and what you have to do to get that brick rolling, your original gearing was probably setup for somewhat smaller diameter tires and the acceleration off of the line is what kills your mileage in the city. If you work to help this then you will possibly hurt the other end a little, then again you might get lucky and see it stay the same.

[rage'nrat638]

there's always the toyota board

[k72516]

rage'nrat638, I need to see video of that truck, NOW! That's insane!!! Post post post post post post please! lol

[bumpsburb]

How do I find out the gears? I am not looking for power, and going up hills the burb really slows down and put my foot in it to uphill. The truck is equiped with the TBI, not sure of the size if it has one. Would it help if I put fuel injection on it? I can assume the power and MPG will increase when I install the headers and 2 1/2 inch pipes on it.

Bump

[Boog]

K72516. As evidenced by my 88 GMC 305, 700R4 and 2.73 gears, there are a lot of trucks leaving the factories with a less than perfect combination of components. Looking at the numbers you might think my short should get real good mileage but 15 city/ 16.5 highway is the best it will do. The 305 needs to be turning more rpms to get better mileage. The 350's, most always, get better mileage. Some exceptions of course. 4 wheel drives and great fuel mileage don't go together.