How would I get better 6 cyl gas mileage? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Pumkin Truck · 10-09-2002, 11:16 PM

K, here is the deal, I got a better job offer that pays hell of a lot better than my current job, to be exact, it is thirty miles away. My six is all stock from the ugly air cleaner to the oil pan, My question is, should I buy a 4 spd tranny to replace my 3 spd chicken leg since it would be all highway driving? I am really debating this. thanks for all the help. Oh yeah would it be a direct bolt on with out modifying my drive shaft? thanks

whateva · 10-09-2002, 11:20 PM

The final drive of a 3 speed and a 4speed are the same. On the highway it will make no difference.

Longhorn Man · 10-09-2002, 11:34 PM

Give her a real good tune up, install HEI, open air filter, (and rejet if needed) play with the timing, get taller rear tires, and keep your foot out of it.

jku72 · 10-10-2002, 09:55 AM

Buy a used 3.07 rear gear and swap it in. They are cheapo, and not too tough to install, and way cheaper than an overdrive trans. That one step will net more mileage than any other trick.
As long as it is a half ton, there is plenty of gears available, since the rear gears were bolt inable from early 60's to late 70's. 10 or 12 bolt axle gears and carriers can be had CHEAP in those 3.07 or 2.76 ratios. If you were rowing a 4 gear, I would recommend the 2.76(2.67?) because of the granny low, but with the three on the tree/six combo I would only go to 3.07:1.
I expect if the engine runs decent, 20+ mpg with the 6. Heck, my suburban with a 350 4 speed get 19mpg with a 3.07. You could beat that with 2 less cylinders.

Pumkin Truck · 10-10-2002, 10:40 PM

Hmm.. thanks everyone, If i remember my truck only got like 11 miles to the gallon at TDC. When I rebuilt the motor, I took apart the intake/exhaust manifolds to paint the intake, I forgot to realign them before i put them back on (stupid! STUPID!) and now I have to take them back off because excess air is getting in with the gas/air combo. about five years ago my dad did the same thing, but he aligned them as best he could, then took it to the machine shop where they resessed it making it as flat as possible. I am thinking about doing that also, Would that mess with my gas milage?

Alaskan · 10-10-2002, 11:15 PM

All good advice, also get a vacuum gauge and test and tune. Tires play a big role in fuel economy, cheap ones tend to have alot of sidewall deflection. I would use a LT rated tire and stay away from the P-Metric. Proper wheel alignment helps and synthetic oils.

Brad · 10-11-2002, 01:10 PM

After having done the 3.07 with my I6 and 3 spd, I wouldn't do it again. In fact I've got another 3.73 and am considering putting that back in. Eventually, I think a 3.42 or so will be exactly what I will do. I would like to put in a T-5, but $$$ prohibits me from doing so. If I could do that with the 3.42, that would be ideal. I really don't get any better gas mileage with the numerically lower gear. I think it's the amount of clutch and gas I have to use to get the truck going from a dead stop negates the highway mileage. Really, for me there wasn't much difference except now I'm very slow off the line. I can go up to 90mph though, which you don't want to do with 3.73!

Actually, I've secretly been considering the V8 conversion (shhhhh!) for down the road because of the cost of 6 performance parts and the fact that some V* guys are getting 20mph like jku is saying. I'm only getting 17mpg! Dang those V*'s!

stelth2002 · 10-11-2002, 03:56 PM

Brad, I have a 67 LWB with a 6 banger in it also. Mine is a good running fresh rebuilt 292. I was just wondering what condition you 250 is in. That would give me more of an idea about the performacen. Because I know the 3.07's would give me another 4 on the highway minium. I have the gears and am still in need to a posi carrier? I would be willing to buy someone's in good condition. Anyways email me what you think the outcome of the 307 would have with a fresh 292. I like the performace it has now, but need to go faster than 50 on highway. Right now im 55 at 2500 with about 30 inch tires.

mikep · 10-11-2002, 05:27 PM

How do you get 415 ft lbs of torque from a 292?

Brad · 10-12-2002, 04:10 AM

Hmmm, it might work for you. I just turned 100,000 miles on the 250. Also, I'm screwing with this Weber 32/36 trying to tune it for a 6. You probably have a little more horses and it might work for you.

GMC71k20 · 10-12-2002, 01:49 PM

get a header

Kas68c10 · 10-15-2002, 08:04 PM

All good suggestions,pretty much what I did. I have 3.73 gears ,a t-5, HEI, headers,electric fan,weber carb with k &n filter, but too small a tire on my truck. Depends how much $ you want to spend. I only get 20 mpg, but then the engine is original, I have never even had the valve cover off it. A rebuild of a 292 is in the plan this winter.
The tune up, rear end gear swap, hei, open air filter are probably what I would do first. A compression test and vacuum test would be a good idea. The t 5 is nice, I rev 1800 @ 60 mph, 2000 @70. Probably off a bit because of the tires, but I love cruising in the truck and being able to have a conversation. Headers didnt seem to do much, but it sounds good. The electric fan I think helped power, not economy.

Hope that helped, and good luck

Brad · 10-21-2002, 07:20 PM

Holy Cow! I got 19.5 mpg doing 65 for 3 hours. I also put more air in the tires too. I've got a tonneau on it also. That is the best mileage I've ever gotten in the truck.

10-09-2002, 11:16 PM	#1
Pumkin Truck Thats just prime! Join Date: Mar 2002 Location: TEXAS Posts: 720	How would I get better 6 cyl gas mileage? K, here is the deal, I got a better job offer that pays hell of a lot better than my current job, to be exact, it is thirty miles away. My six is all stock from the ugly air cleaner to the oil pan, My question is, should I buy a 4 spd tranny to replace my 3 spd chicken leg since it would be all highway driving? I am really debating this. thanks for all the help. Oh yeah would it be a direct bolt on with out modifying my drive shaft? thanks __________________ 69 GMC short bed stepside with a wimpy six that can barely pull itself. 99 Chevrolet Camaro Z-28 1 Bad 5.7ltr LS1

10-10-2002, 09:55 AM	#4
jku72 Registered User Join Date: Jan 2002 Location: cornville, maine Posts: 659	Buy a used 3.07 rear gear and swap it in. They are cheapo, and not too tough to install, and way cheaper than an overdrive trans. That one step will net more mileage than any other trick. As long as it is a half ton, there is plenty of gears available, since the rear gears were bolt inable from early 60's to late 70's. 10 or 12 bolt axle gears and carriers can be had CHEAP in those 3.07 or 2.76 ratios. If you were rowing a 4 gear, I would recommend the 2.76(2.67?) because of the granny low, but with the three on the tree/six combo I would only go to 3.07:1. I expect if the engine runs decent, 20+ mpg with the 6. Heck, my suburban with a 350 4 speed get 19mpg with a 3.07. You could beat that with 2 less cylinders. __________________ jku Cornville, Maine 68 C10 burb 42 Willys MB 46 Dodge WF-32 1.5 ton 53 & 56 Olds' 60 Cadillac 22 Dodge Brothers

10-10-2002, 10:40 PM	#5
Pumkin Truck Thats just prime! Join Date: Mar 2002 Location: TEXAS Posts: 720	Hmm.. thanks everyone, If i remember my truck only got like 11 miles to the gallon at TDC. When I rebuilt the motor, I took apart the intake/exhaust manifolds to paint the intake, I forgot to realign them before i put them back on (stupid! STUPID!) and now I have to take them back off because excess air is getting in with the gas/air combo. about five years ago my dad did the same thing, but he aligned them as best he could, then took it to the machine shop where they resessed it making it as flat as possible. I am thinking about doing that also, Would that mess with my gas milage? __________________ 69 GMC short bed stepside with a wimpy six that can barely pull itself. 99 Chevrolet Camaro Z-28 1 Bad 5.7ltr LS1

10-10-2002, 11:15 PM	#6
Alaskan Truck'n the alcan Join Date: Jan 2001 Location: Houston, AK Posts: 149	All good advice, also get a vacuum gauge and test and tune. Tires play a big role in fuel economy, cheap ones tend to have alot of sidewall deflection. I would use a LT rated tire and stay away from the P-Metric. Proper wheel alignment helps and synthetic oils. __________________ 68 GMC 4x4 71 C20 Custom Deluxe 83 1/2 ton conversion van

10-11-2002, 01:10 PM	#7
Brad Out of the carpool lane. Join Date: Jan 2000 Location: Clark Co, WA Posts: 5,673	After having done the 3.07 with my I6 and 3 spd, I wouldn't do it again. In fact I've got another 3.73 and am considering putting that back in. Eventually, I think a 3.42 or so will be exactly what I will do. I would like to put in a T-5, but $$$ prohibits me from doing so. If I could do that with the 3.42, that would be ideal. I really don't get any better gas mileage with the numerically lower gear. I think it's the amount of clutch and gas I have to use to get the truck going from a dead stop negates the highway mileage. Really, for me there wasn't much difference except now I'm very slow off the line. I can go up to 90mph though, which you don't want to do with 3.73! Actually, I've secretly been considering the V8 conversion (shhhhh!) for down the road because of the cost of 6 performance parts and the fact that some V* guys are getting 20mph like jku is saying. I'm only getting 17mpg! Dang those V's! __________________ 1968 C-10 SWB, 5.7 Vortec/700R4/3.73 posi, Torch Red 1968 Camaro, 250/Powerglide, all original (No, I'm not gonna drop a 350 in it!...Jeez!) 2000 Honda VFR in the faster yellow! 2008 Husqvarna TE-610 1967 C-10 SWB 'Six Appeal'-Gone but not forgotten... Last edited by Brad; 10-11-2002 at 01:16 PM.*

10-09-2002, 11:20 PM	#2
whateva Registered User Join Date: Dec 2001 Location: Poway, Ca Posts: 108	The final drive of a 3 speed and a 4speed are the same. On the highway it will make no difference.

10-09-2002, 11:34 PM	#3
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,693	Give her a real good tune up, install HEI, open air filter, (and rejet if needed) play with the timing, get taller rear tires, and keep your foot out of it.

10-11-2002, 03:56 PM	#8
stelth2002 Young Gun wth Fast Inline Join Date: Jan 2002 Location: Idaho Posts: 1,475	307 conversion Brad, I have a 67 LWB with a 6 banger in it also. Mine is a good running fresh rebuilt 292. I was just wondering what condition you 250 is in. That would give me more of an idea about the performacen. Because I know the 3.07's would give me another 4 on the highway minium. I have the gears and am still in need to a posi carrier? I would be willing to buy someone's in good condition. Anyways email me what you think the outcome of the 307 would have with a fresh 292. I like the performace it has now, but need to go faster than 50 on highway. Right now im 55 at 2500 with about 30 inch tires. __________________ 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!!!

10-11-2002, 05:27 PM	#9
mikep Used to have a truck Join Date: May 2002 Location: port orchard WA Posts: 1,552	How do you get 415 ft lbs of torque from a 292? __________________ No truck :-(

10-12-2002, 04:10 AM	#10
Brad Out of the carpool lane. Join Date: Jan 2000 Location: Clark Co, WA Posts: 5,673	Hmmm, it might work for you. I just turned 100,000 miles on the 250. Also, I'm screwing with this Weber 32/36 trying to tune it for a 6. You probably have a little more horses and it might work for you. __________________ 1968 C-10 SWB, 5.7 Vortec/700R4/3.73 posi, Torch Red 1968 Camaro, 250/Powerglide, all original (No, I'm not gonna drop a 350 in it!...Jeez!) 2000 Honda VFR in the faster yellow! 2008 Husqvarna TE-610 1967 C-10 SWB 'Six Appeal'-Gone but not forgotten...

10-12-2002, 01:49 PM	#11
GMC71k20 Registered User Join Date: Aug 2002 Location: mass Posts: 1,277	get a header

10-15-2002, 08:04 PM	#12
Kas68c10 Registered User Join Date: Apr 2000 Location: Freeland,Michigan,USA Posts: 312	All good suggestions,pretty much what I did. I have 3.73 gears ,a t-5, HEI, headers,electric fan,weber carb with k &n filter, but too small a tire on my truck. Depends how much $ you want to spend. I only get 20 mpg, but then the engine is original, I have never even had the valve cover off it. A rebuild of a 292 is in the plan this winter. The tune up, rear end gear swap, hei, open air filter are probably what I would do first. A compression test and vacuum test would be a good idea. The t 5 is nice, I rev 1800 @ 60 mph, 2000 @70. Probably off a bit because of the tires, but I love cruising in the truck and being able to have a conversation. Headers didnt seem to do much, but it sounds good. The electric fan I think helped power, not economy. Hope that helped, and good luck __________________ 1968 SWB Stepside, 250,pwr disc brake and tilt pwr steering, 5 spd tranny, HEI,Accel coil,K&N filter,Seat from a later truck with built in seatbelts, front and rear sway bars, moved gas tank to under the bed, headman hedders, new chrome bumpers,one wire alternator,Weber k550cub carburator...WORK IN PROGRESS, Freeland MI , Just bought a 72 Jimmy!!!!!!!!!!!

10-21-2002, 07:20 PM	#13
Brad Out of the carpool lane. Join Date: Jan 2000 Location: Clark Co, WA Posts: 5,673	Holy Cow! I got 19.5 mpg doing 65 for 3 hours. I also put more air in the tires too. I've got a tonneau on it also. That is the best mileage I've ever gotten in the truck. __________________ 1968 C-10 SWB, 5.7 Vortec/700R4/3.73 posi, Torch Red 1968 Camaro, 250/Powerglide, all original (No, I'm not gonna drop a 350 in it!...Jeez!) 2000 Honda VFR in the faster yellow! 2008 Husqvarna TE-610 1967 C-10 SWB 'Six Appeal'-Gone but not forgotten...



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