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

oldcarbuff · 11-03-2003, 04:55 PM

I have a 67 half ton long step with the 250 six cylinder and a three speed on the column. I get just under 15 miles per gallon on this thing, which is not a joyous amount, as I am getting divorced, and I hate to see the little money I am going to have left running through the carb quite so quick.

Every thing on this truck is bone stock-I have set the timing carefully, but have done nothing else to it--what suggestions do you all have for improving mileage? HEI distrib? plugs? wires?

If I had a bigger budget, I would love to figure out how to swab a TBI unit from a 4.3 v6 onto the old six banger, but not this year.

My dad has a 72, and he gets 16 plus with an automatic and a 350.

My truck does wind up VERY quickly. I seldom drive over 45-50, because it sounds like it is revving so high. At just a bit over 60, it sounds as if it will (or does) float a valve. Makes an awful noise. The engine is clean (in the valve cover, and side covers), runs like a champ, idles great, no smoke.

OCB

rustyold69 · 11-03-2003, 05:01 PM

i would put some lower gears in the rear end like 3.08s, but you would lose alot of acceleration.

cableguy0 · 11-03-2003, 05:02 PM

most likely that truck has 3.73 gears if you swapped to a set of 3.07's it would lower your rpms greatly on the highway therefore better gas mileage. also get rid of the points ignition and put an hei in there some people have seen as much a 2mog increase with just the hei

68 Suburban · 11-03-2003, 05:04 PM

15 mpg is really not to bad for driving a brick wall. HEI would help. Before you invest a lot of money. Figure out the cost and effect of the change. It really would be silly to spend $1500 to get 1 or 2 mpg better. Unless you do a LOT of driving.

bpmcgee · 11-03-2003, 05:10 PM

I'm not sure that I can agree that putting numerically lower gears in would help. My understanding is that this might help, but may also hurt if you end up bogging the engine down too much.

In these big trucks, everything I read suggests that the i6 and the v8 got just about the SAME mileage because the 6 had to work so much harder to hump the truck down the road. Because of this, I think that putting 3.08s in might actually hurt because the 6 would have a really hard time.

My understanding is that the way to improve MPG is to do things which improve the efficiency of the engine. HEI, as you suggest, free flowing air filters, and possibly headers come to mind.

Also remember that simple things like EGR and PCV valves can affect mileage as well. Make sure they're working right!

Brian

stelth2002 · 11-03-2003, 06:40 PM

I would definatly put an HEI in there. I think that might give you 2 mpg max. more. It sounds like you have her tuned pretty good. Make sure your advance is working right. From my experience most of the non 292 trucks had 4.10 gears. Which would explain why you think your floating valves at 60. I've got a 3.73 posi in mine and it works good. Ive had her over a 100 easy and im pretty sure you could turn 75 if you had to. However you will not get milage goin that fast.
I would aviod messin with gears, unless you can swap rears, and id just put an HEI in it and use it.

**Yukon Jack** · 11-03-2003, 06:44 PM

I have to agree that the mileage your getting is pretty good. Heck my 95 Yukon only gets about 13 mpg and my 69 when I checked when it had the 307 was getting 9 to 10 mpg. Realistically, unless someone comes up with some cheap suggestions, you are going to be money ahead leaving it as it is.

cdowns · 11-03-2003, 06:53 PM

when i switched to a k&n air filter and synthetic(mobil1) oil i picked up over 2mpg, i'd also suggest a gear ratio swap but i think the 3:07's would dog it out something around 3:20's would probably work out good

68 Suburban · 11-03-2003, 07:07 PM

Quote:

Originally posted by cdowns
when i switched to a k&n air filter and synthetic(mobil1) oil i picked up over 2mpg, i'd also suggest a gear ratio swap but i think the 3:07's would dog it out something around 3:20's would probably work out good

Synthetic lubes all the way round might help some more. You can get synthetic for the rear end and the trans too.

oldsub86 · 11-03-2003, 07:51 PM

If you don't mind it being a bit slower in acceleration change the rear end gears as already suggested.

Back in the 70's my dad had a 1968 Ford pickup with a 352 V8 and a 3 speed manual transmission. It was awful on gas. He swapped out the 352 in favour of a 240 I6 and the mileage became incredible. The rear end was obviously intended for the V8. It became more difficult to get the truck moving if it was loaded but it would really cruise on the highway and was doing in the mid 20's on mileage. It wouldn't have been everyone's cup of tea as it was a bit lazy in the acceleration department but the mileage improvement was unreal.

lukecp · 11-03-2003, 07:56 PM

If you don't want to go through the trouble of swapping rear axles....get some bigger tires on the back....it will make it feel like your ratio is a tad higher

I agree....15mpg isn't bad at all out of one of these trucks. Most V8 models get 10-12....i would expect a 5 with a normal driver to get 15-17.

oldcarbuff · 11-04-2003, 10:53 AM

thanks, maybe I just have too high of expectations for the old beast. I do plan to get bigger tires for the back, but the ones on there are nearly new, so I am holding out until they get at least partly worn out.

I may have to look in to new gears for the rear also. I don't know what they are, but they seem to be extremly (numerically) high, as I literally can't go over 60 mph. fortunately, I drive country roads mostly, so 40-45 works fine.

Thanks for all the suggestions. I am going to start looking for an HEI distributor and a non-stock air cleaner. That tiny snorkel can't be helping much.

OCB

11-03-2003, 04:55 PM	#1
oldcarbuff Registered User Join Date: Sep 2002 Location: Muncie, IN U.S.A. Posts: 60	gas mileage suggestions please I have a 67 half ton long step with the 250 six cylinder and a three speed on the column. I get just under 15 miles per gallon on this thing, which is not a joyous amount, as I am getting divorced, and I hate to see the little money I am going to have left running through the carb quite so quick. Every thing on this truck is bone stock-I have set the timing carefully, but have done nothing else to it--what suggestions do you all have for improving mileage? HEI distrib? plugs? wires? If I had a bigger budget, I would love to figure out how to swab a TBI unit from a 4.3 v6 onto the old six banger, but not this year. My dad has a 72, and he gets 16 plus with an automatic and a 350. My truck does wind up VERY quickly. I seldom drive over 45-50, because it sounds like it is revving so high. At just a bit over 60, it sounds as if it will (or does) float a valve. Makes an awful noise. The engine is clean (in the valve cover, and side covers), runs like a champ, idles great, no smoke. OCB

11-03-2003, 05:01 PM	#2
rustyold69 student wrench turner Join Date: Jan 2003 Location: wrightstown, wisconsin Posts: 192	i would put some lower gears in the rear end like 3.08s, but you would lose alot of acceleration. __________________ 1969 Chevy C-10 with 250 L6 2002 GMC Sierra 2500 HD Duramax with banks six gun

11-03-2003, 05:02 PM	#3
cableguy0 Senior Member Join Date: Apr 2000 Location: Delta,Pa Posts: 14,948	most likely that truck has 3.73 gears if you swapped to a set of 3.07's it would lower your rpms greatly on the highway therefore better gas mileage. also get rid of the points ignition and put an hei in there some people have seen as much a 2mog increase with just the hei __________________ Owner of North Point Car Care in Dundalk Md. We specialize in custom exhaust on both modern and classic vehicles. We are a full service auto shop from classics to modern vehicles. Feel free to contact me with questions. I will give a 10% discount to any board member.

11-03-2003, 06:40 PM	#6
stelth2002 Young Gun wth Fast Inline Join Date: Jan 2002 Location: Idaho Posts: 1,475	mileage I would definatly put an HEI in there. I think that might give you 2 mpg max. more. It sounds like you have her tuned pretty good. Make sure your advance is working right. From my experience most of the non 292 trucks had 4.10 gears. Which would explain why you think your floating valves at 60. I've got a 3.73 posi in mine and it works good. Ive had her over a 100 easy and im pretty sure you could turn 75 if you had to. However you will not get milage goin that fast. I would aviod messin with gears, unless you can swap rears, and id just put an HEI in it and use it. __________________ 1967 Chevrolet LWB with built 292! 415 ft. lbs of torque, 4 speed :O with 27% overdrive too, 3.73 Posi. Frame and body done... getting painted!!!

11-03-2003, 06:44 PM	#7
Yukon Jack Post Whore Join Date: Feb 2000 Location: Rose Hill, KS, USA Posts: 12,686	I have to agree that the mileage your getting is pretty good. Heck my 95 Yukon only gets about 13 mpg and my 69 when I checked when it had the 307 was getting 9 to 10 mpg. Realistically, unless someone comes up with some cheap suggestions, you are going to be money ahead leaving it as it is. __________________ 1970 Blazer with a 400 sbc and 4" lift 1980 Pontiac Trans Am, 455 Oldsmobile 2012 Kawasaki Concours 14

11-03-2003, 05:04 PM	#4
68 Suburban Registered User Join Date: Oct 2000 Location: From Chicago, Live in Phoenix, AZ Posts: 6,802	15 mpg is really not to bad for driving a brick wall. HEI would help. Before you invest a lot of money. Figure out the cost and effect of the change. It really would be silly to spend $1500 to get 1 or 2 mpg better. Unless you do a LOT of driving.

11-03-2003, 05:10 PM	#5
bpmcgee Registered User Join Date: Mar 2003 Location: Northern Illinois Posts: 2,181	I'm not sure that I can agree that putting numerically lower gears in would help. My understanding is that this might help, but may also hurt if you end up bogging the engine down too much. In these big trucks, everything I read suggests that the i6 and the v8 got just about the SAME mileage because the 6 had to work so much harder to hump the truck down the road. Because of this, I think that putting 3.08s in might actually hurt because the 6 would have a really hard time. My understanding is that the way to improve MPG is to do things which improve the efficiency of the engine. HEI, as you suggest, free flowing air filters, and possibly headers come to mind. Also remember that simple things like EGR and PCV valves can affect mileage as well. Make sure they're working right! Brian

11-03-2003, 06:53 PM	#8
cdowns Senior Member Join Date: Jan 2003 Location: daytonabeach Posts: 22,956	when i switched to a k&n air filter and synthetic(mobil1) oil i picked up over 2mpg, i'd also suggest a gear ratio swap but i think the 3:07's would dog it out something around 3:20's would probably work out good __________________ 71c-10 350/2004r/4:11 lowered3/4 longbed/dead by hurricane MEANING OF DEATH::::: SOMEBODY ELSE GETS YOUR STUFF DONT BELIEVE EVERYTHING YOU THINK TAKE MY ADVISE;I DON'T USE IT ANYWAY

11-03-2003, 07:51 PM	#10
oldsub86 Registered User Join Date: Dec 2002 Location: Canada Posts: 695	If you don't mind it being a bit slower in acceleration change the rear end gears as already suggested. Back in the 70's my dad had a 1968 Ford pickup with a 352 V8 and a 3 speed manual transmission. It was awful on gas. He swapped out the 352 in favour of a 240 I6 and the mileage became incredible. The rear end was obviously intended for the V8. It became more difficult to get the truck moving if it was loaded but it would really cruise on the highway and was doing in the mid 20's on mileage. It wouldn't have been everyone's cup of tea as it was a bit lazy in the acceleration department but the mileage improvement was unreal. __________________ 1968 Chevy - 292 with a powerglide

11-03-2003, 07:56 PM	#11
lukecp Formerly yellow72custom Join Date: May 2001 Location: Austin, TX Posts: 7,531	If you don't want to go through the trouble of swapping rear axles....get some bigger tires on the back....it will make it feel like your ratio is a tad higher I agree....15mpg isn't bad at all out of one of these trucks. Most V8 models get 10-12....i would expect a 5 with a normal driver to get 15-17. __________________ '72 Chevy C10 Mild 350/TH350/3.07. Ochre/White. Old high school ride. '70 GMC C2500 '62 327 4bbl/SM465/4.56-geared Dana 60. White/White. Project or parts truck. '97 Saturn SL DD. 1.9/5-speed. 40+ highway mpg

11-04-2003, 10:53 AM	#12
oldcarbuff Registered User Join Date: Sep 2002 Location: Muncie, IN U.S.A. Posts: 60	thanks, maybe I just have too high of expectations for the old beast. I do plan to get bigger tires for the back, but the ones on there are nearly new, so I am holding out until they get at least partly worn out. I may have to look in to new gears for the rear also. I don't know what they are, but they seem to be extremly (numerically) high, as I literally can't go over 60 mph. fortunately, I drive country roads mostly, so 40-45 works fine. Thanks for all the suggestions. I am going to start looking for an HEI distributor and a non-stock air cleaner. That tiny snorkel can't be helping much. OCB



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