'70 Suburban fuel milege - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Christopher · 12-08-2002, 08:44 PM

I have a '70 Suburban with a completely stock 292 L6 and 4 speed manual transmission, with towing gears (rpm is around 3000 at 65 mph) I would like to increase my fuel milege, it is aound 14 mpg now driving very carefully at 55 mph. What would be the best way to improve the milege? I would like to have a higher gear ratio so the engine wouldn't be revving so high at highway speeds, but I still want to be able to tow with it (it is a 3/4 ton and tows like a tractor now, I love towing with it) I probbably will put headers on it, any other suggestions? Thanks a bunch, Christopher

cableguy0 · 12-08-2002, 08:46 PM

get yourself an hei dizzy and trash those points.just about the only other thing you could do is get an overdrive tranny

da-burb · 12-08-2002, 10:58 PM

Well it looks like you might have a 4.56 gear ratio in your truck. I used a 31" tire height at 60 mpg and it calcs out to 2965 rpms. Since it is a 3/4 ton it would be real easy to change your center section to a 4.10 ratio. If you do that your 60mph speed would be 2666 rpms. It is a real common ratio and should be easy to find. By the way, I used the calculator at Randy's Ring and Pinion web site.
http://www.ring-pinion.com/home.shtml

And just a note, at 14mpg, you are doing 40% better than i can get in any of my burbs. Face it, they are BRICKS!

stelth2002 · 12-08-2002, 11:11 PM

That L6 is a great engine, also the one that i have and choose. If you want milage you need to get at least 4.10. I have 3.73 POSI and it is still to high for me. As for power there is plenty there for me and I do tow some with a moderate trailer load, a couple thousand pounds up to 4-5k. I am wanting to go with 3.07 ratio. If you put 3.73 in there you would probably get 16 which isn't to bad. Before I rebuilt my truck I got 10-12 with the worn out L6. Now I get 20!.

Christopher · 12-10-2002, 12:57 PM

Thanks guys for the ideas, it looks like I will be putting out some money to get it to where I want it. stelth2002, what did you do to your engine to get that much torque? Thanks again, Chris

jku72 · 12-10-2002, 06:20 PM

If you have the "granny tranny" with the very low first gear, use it, and swap to a 3.07 rear. The ultra low first gear is like 7:1 ratio, more than is needed on anything smaller than a 2 ton truck. I have a 350 4 speed, and swapped in a 3.07, and went from 14 to 19.9 mpg. All this is based on an important assumption, that is that you have the versatile 12 bolt rear, for which 3.07 gears are cheap and plentiful. I did my whole swap for the high price of $75 dollars. I got excellent used gears and a carrier from a bone yard, and they gave me money for my 3.73 with carrier. Most hot rod guys want the 3.73, or 4.10's, so you could trade even or end up making a profit on the deal if you do it right. Now, even with the 3.07, I can tow heavy loads(like a car on a trailer) and still get decent mileage.

12-08-2002, 08:44 PM	#1
Christopher Registered User Join Date: Jul 2002 Posts: 5	'70 Suburban fuel milege I have a '70 Suburban with a completely stock 292 L6 and 4 speed manual transmission, with towing gears (rpm is around 3000 at 65 mph) I would like to increase my fuel milege, it is aound 14 mpg now driving very carefully at 55 mph. What would be the best way to improve the milege? I would like to have a higher gear ratio so the engine wouldn't be revving so high at highway speeds, but I still want to be able to tow with it (it is a 3/4 ton and tows like a tractor now, I love towing with it) I probbably will put headers on it, any other suggestions? Thanks a bunch, Christopher

12-08-2002, 08:46 PM	#2
cableguy0 Senior Member Join Date: Apr 2000 Location: Delta,Pa Posts: 14,948	get yourself an hei dizzy and trash those points.just about the only other thing you could do is get an overdrive tranny __________________ 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.

12-08-2002, 10:58 PM	#3
da-burb "Ochre Ogre" Join Date: Apr 2000 Location: Springfield, MN Posts: 3,558	Well it looks like you might have a 4.56 gear ratio in your truck. I used a 31" tire height at 60 mpg and it calcs out to 2965 rpms. Since it is a 3/4 ton it would be real easy to change your center section to a 4.10 ratio. If you do that your 60mph speed would be 2666 rpms. It is a real common ratio and should be easy to find. By the way, I used the calculator at Randy's Ring and Pinion web site. http://www.ring-pinion.com/home.shtml And just a note, at 14mpg, you are doing 40% better than i can get in any of my burbs. Face it, they are BRICKS! __________________ Bowtie Truck Stop Inc. Mid-West GM Truck Restoration Parts Supplier Your Key Parts, Auto Metal Direct, Dynacorn, and Goodmark dealer. like us @ www.facebook.com/BowtieTruckStop 1971 C-10 Suburban (Ochre) 1971 K-10 Suburban (Ochre) 1972 C-10 Suburban (Ochre) 1972 K-20 Suburban (Yellow- that just aint right!) Springfield, Minnesota 56087

12-08-2002, 11:11 PM	#4
stelth2002 Young Gun wth Fast Inline Join Date: Jan 2002 Location: Idaho Posts: 1,475	milage That L6 is a great engine, also the one that i have and choose. If you want milage you need to get at least 4.10. I have 3.73 POSI and it is still to high for me. As for power there is plenty there for me and I do tow some with a moderate trailer load, a couple thousand pounds up to 4-5k. I am wanting to go with 3.07 ratio. If you put 3.73 in there you would probably get 16 which isn't to bad. Before I rebuilt my truck I got 10-12 with the worn out L6. Now I get 20!. __________________ 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!!!

12-10-2002, 06:20 PM	#6
jku72 Registered User Join Date: Jan 2002 Location: cornville, maine Posts: 659	If you have the "granny tranny" with the very low first gear, use it, and swap to a 3.07 rear. The ultra low first gear is like 7:1 ratio, more than is needed on anything smaller than a 2 ton truck. I have a 350 4 speed, and swapped in a 3.07, and went from 14 to 19.9 mpg. All this is based on an important assumption, that is that you have the versatile 12 bolt rear, for which 3.07 gears are cheap and plentiful. I did my whole swap for the high price of $75 dollars. I got excellent used gears and a carrier from a bone yard, and they gave me money for my 3.73 with carrier. Most hot rod guys want the 3.73, or 4.10's, so you could trade even or end up making a profit on the deal if you do it right. Now, even with the 3.07, I can tow heavy loads(like a car on a trailer) and still get decent mileage. __________________ jku Cornville, Maine 68 C10 burb 42 Willys MB 46 Dodge WF-32 1.5 ton 53 & 56 Olds' 60 Cadillac 22 Dodge Brothers

12-10-2002, 12:57 PM	#5
Christopher Registered User Join Date: Jul 2002 Posts: 5	Thanks guys for the ideas, it looks like I will be putting out some money to get it to where I want it. stelth2002, what did you do to your engine to get that much torque? Thanks again, Chris



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