gas mileage - The 1947 - Present Chevrolet & GMC Truck Message Board Network

J/D/S · 02-23-2010, 09:35 PM

Here's the deal, I want a big block in my 79 short bed I like the way it fills the engine bay up but I want to know what kind of gas mileage I can get? Here's the combo I'm looking to run, a carburated stock 396 or 454 a 700R4 and 308 gears what do you think it will get guys? And please just answer my question no flaming or dumb a$$ remarks.

Matt Man · 02-23-2010, 10:01 PM

from what i have read you can not see past 12mpg in a big block period. i get about 10. but your truck is lighter so maybe u can see a bit better than 12. how tall of a tire with those 3:08's?

J/D/S · 02-23-2010, 10:07 PM

I was thinking of putting 20" rims on her

Benelli · 02-23-2010, 10:09 PM

I suggest going with 3.42 gears. I think that it would let the motor be under less load, which could use less gas. Then again, I'm not sure on the difference between a SB and a BB.

Big Port Jimmy 6 · 02-23-2010, 10:35 PM

With the 700 and a 336 or a 342 gear you could get in the low teens. A 308 is too high in my opinion in overdrive. It all depends on the cam selection and the other specs of the big block. My TBI 454/400 only gets around 6\7 in town with short tires and a 308. I am pulling it out for a 5.3\4l60E combo. Jay

spinem · 02-23-2010, 10:39 PM

yyuck yyuck iz a big block!!! no good gas milage!! yuck. oh sorry, no smart a$$...
11 keeping with trafic in town. 14hw....15/w/ the motor off rolling downhill (couldent help it)

J/D/S · 02-25-2010, 05:02 PM

Why 342 gears? they'll make the engine work harder then 308's

munger · 02-25-2010, 10:03 PM

342s will keep the motor in a sweet spot where it will pull good in od and get good mileage 308s will just lugg it causing worse gas mileage and no power at all in od

Big Port Jimmy 6 · 02-25-2010, 10:02 PM

With an over drive the 3:42 gears are actually 2.39 in over drive the 308s are 2.15. The 342 will also help you come off the light a little easier. I usually like a 3:73 (2.61) with the overdrives, but with the torque of the big block you will be ok a little taller gear. Jay

J/D/S · 02-25-2010, 10:49 PM

So what will the final ratio work out to with 20" rims and 308's. And can anyone tell me what the cruising RPM of a stock big block is at 75 MPH with a 700R4 and low gears like 342 or lower

Raised On Chevy! · 02-25-2010, 11:03 PM

The size of the rim isn't what you really need to know, it's the over all size of the rear tires, that will tell what you need for gears. As for the big block thing, if your looking for milage your looking at the wrong engine!

kenm78 · 02-25-2010, 11:10 PM

You didn't say what side tire you wanted just wheel, Assuming your stock tire is a 235/75/R15 which most 1/2 tons were. I figured using a 275/40R20 tire you get a final drive of around 2.16:1

longbedrick · 02-26-2010, 09:49 AM

back when the '73 LWB was my daily driver I was getting about 7mpg. I was getting about 10 after a full tune up with a rebuilt HEI distributor and newer tires. My dad's old fuel injected 454 dually got about 10mpg

larrystone · 02-26-2010, 10:07 AM

My 1992 came with the 700r4, 4.3 tbi and 3.08 rear.

When it was new I got 25 mpg with a 500 pound motorcycle in an open bed.

If you want gas mileage with acceptable power I suggest you pull a 4.3 with the fancy fuel injection and the computer from a wrecked small S-10 Chevy Blazer.

grinch4om · 02-26-2010, 10:55 AM

I get 13-14 on the highway...10-12 in town, warmer weather the in town improves a tad

02-23-2010, 09:35 PM	#1
J/D/S Registered User Join Date: Jan 2008 Location: Fort Campbell KY Posts: 271	gas mileage Here's the deal, I want a big block in my 79 short bed I like the way it fills the engine bay up but I want to know what kind of gas mileage I can get? Here's the combo I'm looking to run, a carburated stock 396 or 454 a 700R4 and 308 gears what do you think it will get guys? And please just answer my question no flaming or dumb a$$ remarks. __________________ John Bernard Books (The Shootist): "I won't be wronged, I won't be insulted, and I won't be laid a hand on. I don't do these things to other people and I expect the same from them."

02-23-2010, 10:01 PM	#2
Matt Man Registered User Join Date: Oct 2009 Location: Buhl, Idaho Posts: 1,438	Re: gas mileage from what i have read you can not see past 12mpg in a big block period. i get about 10. but your truck is lighter so maybe u can see a bit better than 12. how tall of a tire with those 3:08's? __________________ 86 c30 crew cab dually 454/400 4/6 drop SOLD 1-8-11 1981 chevy K10 short Bed Scottsdale SOLD 10-26-2020 67 GMC Stepside Straight six/3 on the tree "The Peach" 2020 Silverado Double Cab LT Z71 5.3 8 speed Daily Driver K10 build http://67-72chevytrucks.com/vboard/s...d.php?t=671934 The Peach build http://67-72chevytrucks.com/vboard/s...d.php?t=733903

02-23-2010, 10:07 PM	#3
J/D/S Registered User Join Date: Jan 2008 Location: Fort Campbell KY Posts: 271	Re: gas mileage I was thinking of putting 20" rims on her __________________ John Bernard Books (The Shootist): "I won't be wronged, I won't be insulted, and I won't be laid a hand on. I don't do these things to other people and I expect the same from them."

02-23-2010, 10:09 PM	#4
Benelli Watchin the rust grow Join Date: Oct 2008 Location: Western Nebraska Posts: 950	Re: gas mileage I suggest going with 3.42 gears. I think that it would let the motor be under less load, which could use less gas. Then again, I'm not sure on the difference between a SB and a BB. __________________ 1979 Chevy K10 (H/D Front end) - 350 w/ Holley Truck Avenger, Weinald Intake manifold, headers, bored .060 over, mild cam, Built up TH350 (TranStar Shift kit) with a part-timed NP203 t-case SOLD ($3,000) 1997 Chevy Silverado 1500 - 4.3 vortec, extended cab, 21 highway MPG, stock. "Red" SOLD ($3,000) 1987 Toyota 4wd Pick-up - work in progress, future DD SOLD ($1,850) 1998 Dodge Ram 2500 Cummins 5spd Quad cab - old 12 valve with some work done 2004 Chevy Silverado - Black, 4x4, 5.3L, stock for now Underneath that rust is a heart of gold

02-23-2010, 10:35 PM	#5
Big Port Jimmy 6 Addicted to Rust Join Date: Dec 2009 Location: Abbeville SC Posts: 1,030	Re: gas mileage With the 700 and a 336 or a 342 gear you could get in the low teens. A 308 is too high in my opinion in overdrive. It all depends on the cam selection and the other specs of the big block. My TBI 454/400 only gets around 6\7 in town with short tires and a 308. I am pulling it out for a 5.3\4l60E combo. Jay __________________ Current trucks: (48 GMC100 320 inliner) Gone (48 Suburban in pieces) Sold 87 GMC CCSB 5.3 4l60E Daily Driver ECTA National record holder XO/PP We never really grow up, we just learn how to act in public.

02-23-2010, 10:39 PM	#6
spinem haha, ford guys... Join Date: Feb 2007 Location: Visalia, CA Posts: 576	Re: gas mileage yyuck yyuck iz a big block!!! no good gas milage!! yuck. oh sorry, no smart a$$... 11 keeping with trafic in town. 14hw....15/w/ the motor off rolling downhill (couldent help it)

02-25-2010, 05:02 PM	#7
J/D/S Registered User Join Date: Jan 2008 Location: Fort Campbell KY Posts: 271	Re: gas mileage Why 342 gears? they'll make the engine work harder then 308's __________________ John Bernard Books (The Shootist): "I won't be wronged, I won't be insulted, and I won't be laid a hand on. I don't do these things to other people and I expect the same from them."

02-25-2010, 10:03 PM	#8
munger Registered User Join Date: Nov 2009 Location: goodland,ks Posts: 217	Re: gas mileage 342s will keep the motor in a sweet spot where it will pull good in od and get good mileage 308s will just lugg it causing worse gas mileage and no power at all in od __________________ mitch 87 chevy short box 4wd tbi 350/built 700r4/np208 3.73s

02-25-2010, 10:02 PM	#9
Big Port Jimmy 6 Addicted to Rust Join Date: Dec 2009 Location: Abbeville SC Posts: 1,030	Re: gas mileage With an over drive the 3:42 gears are actually 2.39 in over drive the 308s are 2.15. The 342 will also help you come off the light a little easier. I usually like a 3:73 (2.61) with the overdrives, but with the torque of the big block you will be ok a little taller gear. Jay __________________ Current trucks: (48 GMC100 320 inliner) Gone (48 Suburban in pieces) Sold 87 GMC CCSB 5.3 4l60E Daily Driver ECTA National record holder XO/PP We never really grow up, we just learn how to act in public.

02-25-2010, 10:49 PM	#10
J/D/S Registered User Join Date: Jan 2008 Location: Fort Campbell KY Posts: 271	Re: gas mileage So what will the final ratio work out to with 20" rims and 308's. And can anyone tell me what the cruising RPM of a stock big block is at 75 MPH with a 700R4 and low gears like 342 or lower __________________ John Bernard Books (The Shootist): "I won't be wronged, I won't be insulted, and I won't be laid a hand on. I don't do these things to other people and I expect the same from them."

02-25-2010, 11:03 PM	#11
Raised On Chevy! Registered User Join Date: Feb 2010 Location: Revelstoke BC Canada Posts: 98	Re: gas mileage The size of the rim isn't what you really need to know, it's the over all size of the rear tires, that will tell what you need for gears. As for the big block thing, if your looking for milage your looking at the wrong engine!

02-25-2010, 11:10 PM	#12
kenm78 Broken Down Join Date: Dec 2006 Location: Choctaw, Oklahoma Posts: 1,267	Re: gas mileage You didn't say what side tire you wanted just wheel, Assuming your stock tire is a 235/75/R15 which most 1/2 tons were. I figured using a 275/40R20 tire you get a final drive of around 2.16:1 __________________ kenm78 1982 Chevy 4X4 Project truck “It’s better to keep your mouth shut and let people wonder about your ignorance, than open your mouth and reveal all doubt”

02-26-2010, 09:49 AM	#13
longbedrick Registered User Join Date: Nov 2009 Location: Gilroy,CA Posts: 193	Re: gas mileage back when the '73 LWB was my daily driver I was getting about 7mpg. I was getting about 10 after a full tune up with a rebuilt HEI distributor and newer tires. My dad's old fuel injected 454 dually got about 10mpg

02-26-2010, 10:07 AM	#14
larrystone Registered User Join Date: Jan 2008 Location: White Haven PA Posts: 59	Re: gas mileage My 1992 came with the 700r4, 4.3 tbi and 3.08 rear. When it was new I got 25 mpg with a 500 pound motorcycle in an open bed. If you want gas mileage with acceptable power I suggest you pull a 4.3 with the fancy fuel injection and the computer from a wrecked small S-10 Chevy Blazer. __________________ 1992 C1500 WT LWB 4.3TBI 700r4 4.10 limited slip= Work Truck 1986 C10 Custom Deluxe LWB 4.3-4V TH350C 2.73 ac cruise tilt= Cowboy Cadillac

02-26-2010, 10:55 AM	#15
grinch4om dang its hot Join Date: Nov 2006 Location: frankfort kentucky Posts: 1,349	Re: gas mileage I get 13-14 on the highway...10-12 in town, warmer weather the in town improves a tad



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