C20 milage improvements - The 1947 - Present Chevrolet & GMC Truck Message Board Network

swamp rat · 10-25-2007, 01:06 AM

After reading the thread for C10 milage I started to think about my truck and how bad the milage is and how much better the milage is in the newer trucks.

I am left to wonder what other 3/4T owners have done to improve milage?

Different gearing, Tranny, running gear or engine mods? Fuel injection or a newer engine?

Has anybody here ever yanked a body off of a late model truck and mounted the classic body style into it?

ridin98ci · 10-25-2007, 01:12 AM

Im there with u. I have a 69 3/4 ton and am getting about 9mpg. I would be happy with 12-14mpg.

custom72c20 · 10-25-2007, 01:17 AM

Quote:

Originally Posted by ridin98ci

Im there with u. I have a 69 3/4 ton and am getting about 9mpg. I would be happy with 12-14mpg.

same here i drive a 72 c20. its always thirsty. i really feel it when i go to fill up.

brianthelion02 · 10-25-2007, 01:26 AM

My k-20 always got 8 mpg,went to a 700r4 and went to a solid 13mpg!!!!

**WorkinLonghorn** · 10-25-2007, 01:32 AM

I've got a heavy (5200 lb.) longhorn 3/4 and I get 12 MPG no matter what I haul or pretty much,how I drive. But it's a stick and it is running about as lean as it gets,plus an adjustable vac-advance on the dizzy,dialed in.

ridin98ci · 10-25-2007, 01:40 AM

Quote:

Originally Posted by brianthelion02

My k-20 always got 8 mpg,went to a 700r4 and went to a solid 13mpg!!!!

Im new to the chevy game.....mopar convert.....how would I go about swaping in a 700R4 for the grannie low 4 speed in her now?
James

**WorkinLonghorn** · 10-25-2007, 02:11 AM

ridin98ci,I don't think it's practical.You'd need a bunch of parts (trans+torque converter,collumn shift or floor shifter,cross-member,cooling lines kick-down linkage,radiator with cooler,modulator line etc) just to get a little better mileage,and I don't think his gains are at all typical.If you need to lower your cruise RPM you can get taller gears in rear end. I get 12 average with 3.73 rear and a granny 4 spd.

davidh · 10-25-2007, 02:52 AM

Most 3/4 tons with the 6.0 engine are only getting 13mpg on average. I think that can be matched.

mrein3 · 10-25-2007, 08:20 AM

First thing I would do is figure out the gear ratios in your axels. The chances are they are 373s. If they are 308s, going to an overdrive transmission won't buy you much. In fact some here have posted that 308s with OD HURTS their fuel mileage.

If you have 373s or 411s, a 700r4 upgrade may help. You need to locate a core 700r4, buy the Advanced Adapters 700r4 in place of th350 output shaft, and take them to a rebuilder you know and trust.

Pull your engine. Remove the th350 from the transfer case and adapter. Move the engine cradle up. Bolt in the 700r4. Bolt the engine in its new location - approximately 2 inches closer to the radiator. Deal with the fan shroud. Buy a TV made EZ 700r4 kick down cable kit from Bowtie Overdrives. Set it up correctly.

Those mods, about $1500 - $2000 worth, will net you APPROXIMATELY 30% increase in fuel mileage.

But before you do all that, figure out what you spend on fuel per day/month/year. Then calculate what 30% (2.7 miles per gallon if you are at 9 mpg now) will save you. You may find that the payback is 10 years down the road.

Let me put it this way. I get 9.5 mpg in my 1/2 ton, 383, th350 4x4. The payback for a possible 30% increase in fuel mileage would be WAY down the road. So far down the road that so far I've left the th350 in my truck.

I had to drive the truck to work today because my daily driver is on jack stands waiting for parts. I go 38 miles one way, most of it in a 70mph zone.
I'd love to reduce my highway rpms by 30%. But for the amount of money I'd have to sink in a 700r4, needing to replace one daily driver, wife, kid, another kid on the way, justifying a $1500 expense is pretty tough in my case.

Do the math for yourself. Then decide if that potential 30% increase is worth it.

ccd3 · 10-25-2007, 09:42 AM

Smart math- you make some really good points.

ridin98ci · 10-25-2007, 10:56 AM

Yep the math makes sense.....looks like Im gonna be in the slow lane and hittin all the gas stations for a long time!!

Brad · 10-25-2007, 11:30 AM

While mrein3's math is basically correct, what's missing in his statement is that the price of gas will only increase. That means that the payback won't be as far off in the future as one might think!

Lemield · 10-25-2007, 11:33 AM

My C20 with a 454 gets around 9 MPG....I've tried everything I can think of....no real improvement...I did go from a 750 CFM edelbrock carb to a 650 CFM...mileage actually went down a little...my newest solution....I leave it in the driveway unless I'm hauling something....helps a little.

As my buddy says...'It's up to us to burn up the fossil fuels so that they will be forced to find something better' I feel that I'm doing my part....

rlm316 · 10-25-2007, 12:53 PM

Taller tires will cut down the engine RPMs. Fewer turns to make the same distance. Just need to do the math to convert the posted speed to the actual speed. Do a good tune-up. Make sure you are getting as much power out of the motor as there is available. My $.02

heyjoe · 10-25-2007, 01:25 PM

My '67 C20 250ci, 3 on the tree, 4.57 rear gets 10 mpg city.

Old Chevy Mech · 10-25-2007, 01:26 PM

My 70 K-5 gets 15+ on the highway. The engine was built for the truck in 1993 and has most of the curve set in the cam, has oversize dia tires, not over width. It has a carb mod called a "carburator enhancer" installed and I run a Slick 50'd engine & drive train wit Amsoil 10W-40 engine oil. The speedometer is 8-9mph off/low side. Truck has a vacuum gage and I use it to drive by as much as my tach & speedometer. Normal cruise RPM is @ 2500. Adjustable advance set "just shy of ping on a long hill" LOL, K&N air filter, MSD 50K volt coil. Points ignition.

That 15MPG has been verified by 3 people to make sure I wasnt screwed in my calculations.

Frank

CrashBob · 10-25-2007, 02:24 PM

My '67 has a 327 TH400 and 4:56 gears and it doesn't see much speed above 65 as I can't afford a new engine. Mileage was 8-9 mpg depending on the wind direction. Put in a new billet HEI dizzy, 8.5 mm wires and hot coil. Now i'm averaging 13-14 mpg. When the money is available, I'm going with a 700R4 and 3:90 gears.
Try the HEI, worth the bucks, I have about $175 in mine. Good luck!

CrashBob

**tkeduk8r** · 10-25-2007, 04:21 PM

amen to what has allready been posted.... taking the time to dial in your distributor and carb can make a bigger difference than you think... There are some great FAQ's in the FAQ section (I cant remember who by) about tuning each... I have actually used them in handouts I gave to my high school students when I get to ignition and tune up in their autos course... A vacuum gauge and dial back timing light are about the best tuning tools I own, but I would like to try a air/fuel meter also... I have a 305/2004r/3.73/31x10.5's in a 74 C10 and I can hit 19-20mpg driving 55-60... 17-18mpg at 70+ routinely... Board member Green Machine has done some cool things related to tuning and milage, maybe they will chime in here too...
--Mike

swamp rat · 10-25-2007, 04:45 PM

Quote:

Originally Posted by brianthelion02

My k-20 always got 8 mpg,went to a 700r4 and went to a solid 13mpg!!!!

Now were talking! Do you happen to know what gearing you have?

I seem to average 8 last time I checked, I could live with 13 too!

swamp rat · 10-25-2007, 04:52 PM

Quote:

Originally Posted by mrein3

If you have 373s or 411s, a 700r4 upgrade may help. You need to locate a core 700r4, buy the Advanced Adapters 700r4 in place of th350 output shaft, and take them to a rebuilder you know and trust.

Pull your engine. Remove the th350 from the transfer case and adapter. Move the engine cradle up. Bolt in the 700r4. Bolt the engine in its new location - approximately 2 inches closer to the radiator. Deal with the fan shroud. Buy a TV made EZ 700r4 kick down cable kit from Bowtie Overdrives. Set it up correctly.

Those mods, about $1500 - $2000 worth, will net you APPROXIMATELY 30% increase in fuel mileage.

Do the math for yourself. Then decide if that potential 30% increase is worth it.

Is it worth it right now? no but mabey when I decide to do the frame off resto it will be. I do have 3.73:1 gearing. If you have knowledge of any reading for this mod on line I would like to read it. Thanks!!

**67 cst swb** · 10-25-2007, 04:54 PM

In one of my previous pick-ups (a '67 ShortWide 1/2t 2wd) I had a moreless stock 283 w/TH350/3.73 gears and a tall 255/70-15 tire... I got 18-19 on the highway... On a trip once from Wichita, Kansas to Northern Dallas, Texas I got just over 20mph (343 miles on 16.8 gallons, don't ask how I remember that). It did have HEI and a tuned carter 600cfm.

JAKES 68GMC · 10-25-2007, 04:55 PM

I'd kill for 13mpg.

swamp rat · 10-25-2007, 05:00 PM

Quote:

Originally Posted by CrashBob

My '67 has a 327 TH400 and 4:56 gears and it doesn't see much speed above 65 as I can't afford a new engine. Mileage was 8-9 mpg depending on the wind direction. Put in a new billet HEI dizzy, 8.5 mm wires and hot coil. Now i'm averaging 13-14 mpg. When the money is available, I'm going with a 700R4 and 3:90 gears.
Try the HEI, worth the bucks, I have about $175 in mine. Good luck!

CrashBob

Thanks for the info, I have been thinking about this also, I have a mallory Unilite with a Promaster coil, still no help..

swamp rat · 10-25-2007, 05:05 PM

Quote:

Originally Posted by tkeduk8r

amen to what has allready been posted.... taking the time to dial in your distributor and carb can make a bigger difference than you think... There are some great FAQ's in the FAQ section (I cant remember who by) about tuning each... I have actually used them in handouts I gave to my high school students when I get to ignition and tune up in their autos course... A vacuum gauge and dial back timing light are about the best tuning tools I own, but I would like to try a air/fuel meter also... I have a 305/2004r/3.73/31x10.5's in a 74 C10 and I can hit 19-20mpg driving 55-60... 17-18mpg at 70+ routinely... Board member Green Machine has done some cool things related to tuning and milage, maybe they will chime in here too...
--Mike

Yep all good info, I have tuned till I cant tune no more still hovering at 8-9 mpg, but will admit that its time to do it again! I'll look in the FAQ section in the near future.

swamp rat · 10-25-2007, 05:08 PM

Quote:

Originally Posted by rlm316

Taller tires will cut down the engine RPMs. Fewer turns to make the same distance. Just need to do the math to convert the posted speed to the actual speed. Do a good tune-up. Make sure you are getting as much power out of the motor as there is available. My $.02

I run 33's, at 40 I am around 23-2400, at 60 I am around 2700 (all by memory) and without wheel size corrections to the odometer.

10-25-2007, 01:06 AM	#1
swamp rat Registered User Join Date: Oct 2007 Location: Spanaway Posts: 8,451	C20 milage improvements After reading the thread for C10 milage I started to think about my truck and how bad the milage is and how much better the milage is in the newer trucks. I am left to wonder what other 3/4T owners have done to improve milage? Different gearing, Tranny, running gear or engine mods? Fuel injection or a newer engine? Has anybody here ever yanked a body off of a late model truck and mounted the classic body style into it? __________________ Mike. Swamp Rat build thread : http://67-72chevytrucks.com/vboard/s...d.php?t=595019 72 3/4T 4X4 4" BDS Lift 33" BFG's

10-25-2007, 01:12 AM	#2
ridin98ci Registered User Join Date: Oct 2007 Location: Monticello IL Posts: 29	Re: C20 milage improvements Im there with u. I have a 69 3/4 ton and am getting about 9mpg. I would be happy with 12-14mpg.

10-25-2007, 01:26 AM	#4
brianthelion02 Overdrive club Join Date: Sep 2007 Location: apache junction, arizona Posts: 819	Re: C20 milage improvements My k-20 always got 8 mpg,went to a 700r4 and went to a solid 13mpg!!!! __________________ 1971 c-10 1951 3100 chevy 1970 chevy c-30 Sold!

10-25-2007, 01:32 AM	#5
WorkinLonghorn Senior Member Join Date: Dec 2006 Location: Studio City, Calif. Posts: 2,886	Re: C20 milage improvements I've got a heavy (5200 lb.) longhorn 3/4 and I get 12 MPG no matter what I haul or pretty much,how I drive. But it's a stick and it is running about as lean as it gets,plus an adjustable vac-advance on the dizzy,dialed in. __________________ '69 GMC C2500 Custom Camper, 8 1/2' bed, New GM 350, NP 435 Close Ratio 4spd. Trans., 3.73 Dana-60 open.Camper and Trailer wiring, PS, PB, AC, tach , three gas tanks, stereo speakers, 2nd owner, Work-Truck supreme.

10-25-2007, 02:11 AM	#7
WorkinLonghorn Senior Member Join Date: Dec 2006 Location: Studio City, Calif. Posts: 2,886	Re: C20 milage improvements ridin98ci,I don't think it's practical.You'd need a bunch of parts (trans+torque converter,collumn shift or floor shifter,cross-member,cooling lines kick-down linkage,radiator with cooler,modulator line etc) just to get a little better mileage,and I don't think his gains are at all typical.If you need to lower your cruise RPM you can get taller gears in rear end. I get 12 average with 3.73 rear and a granny 4 spd. __________________ '69 GMC C2500 Custom Camper, 8 1/2' bed, New GM 350, NP 435 Close Ratio 4spd. Trans., 3.73 Dana-60 open.Camper and Trailer wiring, PS, PB, AC, tach , three gas tanks, stereo speakers, 2nd owner, Work-Truck supreme.

10-25-2007, 02:52 AM	#8
davidh Senior Member Join Date: Feb 2004 Location: Ponca city now kingfisher Oklahoma Posts: 1,220	Re: C20 milage improvements Most 3/4 tons with the 6.0 engine are only getting 13mpg on average. I think that can be matched. __________________ Even the nicest dodge-chrysler is only 2years away from being parked outside of Section 8 housing.

10-25-2007, 08:20 AM	#9
mrein3 Registered User Join Date: Feb 2000 Location: Center City, MN, USA Posts: 3,254	Re: C20 milage improvements First thing I would do is figure out the gear ratios in your axels. The chances are they are 373s. If they are 308s, going to an overdrive transmission won't buy you much. In fact some here have posted that 308s with OD HURTS their fuel mileage. If you have 373s or 411s, a 700r4 upgrade may help. You need to locate a core 700r4, buy the Advanced Adapters 700r4 in place of th350 output shaft, and take them to a rebuilder you know and trust. Pull your engine. Remove the th350 from the transfer case and adapter. Move the engine cradle up. Bolt in the 700r4. Bolt the engine in its new location - approximately 2 inches closer to the radiator. Deal with the fan shroud. Buy a TV made EZ 700r4 kick down cable kit from Bowtie Overdrives. Set it up correctly. Those mods, about $1500 - $2000 worth, will net you APPROXIMATELY 30% increase in fuel mileage. But before you do all that, figure out what you spend on fuel per day/month/year. Then calculate what 30% (2.7 miles per gallon if you are at 9 mpg now) will save you. You may find that the payback is 10 years down the road. Let me put it this way. I get 9.5 mpg in my 1/2 ton, 383, th350 4x4. The payback for a possible 30% increase in fuel mileage would be WAY down the road. So far down the road that so far I've left the th350 in my truck. I had to drive the truck to work today because my daily driver is on jack stands waiting for parts. I go 38 miles one way, most of it in a 70mph zone. I'd love to reduce my highway rpms by 30%. But for the amount of money I'd have to sink in a 700r4, needing to replace one daily driver, wife, kid, another kid on the way, justifying a $1500 expense is pretty tough in my case. Do the math for yourself. Then decide if that potential 30% increase is worth it. __________________ '70 cab, '71 chassis, 383, TH350, NP205. '71 Malibu convertible '72 Malibu hard top Center City, MN

10-25-2007, 09:42 AM	#10
ccd3 Registered User Join Date: Mar 2007 Location: Austin, TX Posts: 93	Re: C20 milage improvements Smart math- you make some really good points.

10-25-2007, 10:56 AM	#11
ridin98ci Registered User Join Date: Oct 2007 Location: Monticello IL Posts: 29	Re: C20 milage improvements Yep the math makes sense.....looks like Im gonna be in the slow lane and hittin all the gas stations for a long time!!

10-25-2007, 11:30 AM	#12
Brad Out of the carpool lane. Join Date: Jan 2000 Location: Clark Co, WA Posts: 5,672	Re: C20 milage improvements While mrein3's math is basically correct, what's missing in his statement is that the price of gas will only increase. That means that the payback won't be as far off in the future as one might think! __________________ 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...

10-25-2007, 11:33 AM	#13
Lemield Registered User Join Date: Dec 2004 Location: Arvada Colorado Posts: 1,321	Re: C20 milage improvements My C20 with a 454 gets around 9 MPG....I've tried everything I can think of....no real improvement...I did go from a 750 CFM edelbrock carb to a 650 CFM...mileage actually went down a little...my newest solution....I leave it in the driveway unless I'm hauling something....helps a little. As my buddy says...'It's up to us to burn up the fossil fuels so that they will be forced to find something better' I feel that I'm doing my part.... __________________ Larry 1949 Willys CJ2A, Kubota Turbo Diesel engine, Warn Overdrive, 11 inch brakes, 12volt, Warn M8000 Winch "Little Green" 1970 Rustoleum Roller Red C20 Custom Camper 454 TH400 "Big Red" 1971 K20 6.2 Turbo Diesel 4x4 4spd Ranger Overdrive "Pumpkin Wagon" 2005 VW Beetle Diesel 44+ MPG "Silver Bullet"

10-25-2007, 12:53 PM	#14
rlm316 Registered User Join Date: Aug 2006 Location: Woodstown, NJ Posts: 669	Re: C20 milage improvements Taller tires will cut down the engine RPMs. Fewer turns to make the same distance. Just need to do the math to convert the posted speed to the actual speed. Do a good tune-up. Make sure you are getting as much power out of the motor as there is available. My $.02

10-25-2007, 01:25 PM	#15
heyjoe Registered User Join Date: Apr 2003 Location: New Orleans area Posts: 198	Re: C20 milage improvements My '67 C20 250ci, 3 on the tree, 4.57 rear gets 10 mpg city.

10-25-2007, 01:26 PM	#16
Old Chevy Mech Keepin it runnin! Join Date: Oct 2007 Location: Deep River, CT Posts: 1,036	Re: C20 milage improvements My 70 K-5 gets 15+ on the highway. The engine was built for the truck in 1993 and has most of the curve set in the cam, has oversize dia tires, not over width. It has a carb mod called a "carburator enhancer" installed and I run a Slick 50'd engine & drive train wit Amsoil 10W-40 engine oil. The speedometer is 8-9mph off/low side. Truck has a vacuum gage and I use it to drive by as much as my tach & speedometer. Normal cruise RPM is @ 2500. Adjustable advance set "just shy of ping on a long hill" LOL, K&N air filter, MSD 50K volt coil. Points ignition. That 15MPG has been verified by 3 people to make sure I wasnt screwed in my calculations. Frank __________________ 1970 K-5 4X4 Blazer 1970 K-20 4X4 Long Bed "Liberty Means Responsibility, that is why most men dread it"

10-25-2007, 02:24 PM	#17
CrashBob Registered User Join Date: Jun 2004 Location: Utica Nebraska Posts: 244	Re: C20 milage improvements My '67 has a 327 TH400 and 4:56 gears and it doesn't see much speed above 65 as I can't afford a new engine. Mileage was 8-9 mpg depending on the wind direction. Put in a new billet HEI dizzy, 8.5 mm wires and hot coil. Now i'm averaging 13-14 mpg. When the money is available, I'm going with a 700R4 and 3:90 gears. Try the HEI, worth the bucks, I have about $175 in mine. Good luck! CrashBob __________________ "Life is all about ass. You're either covering it,laughing it off, kicking it, kissing it, busting it or trying to get a piece of it.



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10-25-2007, 04:21 PM	#18
tkeduk8r Senior Member Join Date: Apr 2001 Location: Alexandria, MN Posts: 1,462	Re: C20 milage improvements amen to what has allready been posted.... taking the time to dial in your distributor and carb can make a bigger difference than you think... There are some great FAQ's in the FAQ section (I cant remember who by) about tuning each... I have actually used them in handouts I gave to my high school students when I get to ignition and tune up in their autos course... A vacuum gauge and dial back timing light are about the best tuning tools I own, but I would like to try a air/fuel meter also... I have a 305/2004r/3.73/31x10.5's in a 74 C10 and I can hit 19-20mpg driving 55-60... 17-18mpg at 70+ routinely... Board member Green Machine has done some cool things related to tuning and milage, maybe they will chime in here too... --Mike __________________ 63 C20 2WD, 230/SM420/4.11 73K 68 C30 dump truck, 327,SM425,4.56 66K original 68 C10 2WD SWB Stepper, 250/3-tree/3.73 66K.... Wifes 68 Buick Skylark Custom Conv., 350/TH400/2.56 In our family since '79 69 GMC 1500 SWB 2WD, 350/TH350/3.73 posi... 70 K10 SWB 4X4 4-sp,ps, pb, tilt, buckets, console, sliding back window (Need AC parts) WIP 72 Chevy K20 4x4 350/350/205/4.56 donor 72 Vette 350/M21/3.70 posi, triple black and super fast 84 Olds Cutlass Brougham 350/2004r/4.11 was Dad's

10-25-2007, 04:54 PM	#21
67 cst swb Senior Member Join Date: Nov 2001 Location: Rapid City, SD Posts: 2,281	Re: C20 milage improvements In one of my previous pick-ups (a '67 ShortWide 1/2t 2wd) I had a moreless stock 283 w/TH350/3.73 gears and a tall 255/70-15 tire... I got 18-19 on the highway... On a trip once from Wichita, Kansas to Northern Dallas, Texas I got just over 20mph (343 miles on 16.8 gallons, don't ask how I remember that). It did have HEI and a tuned carter 600cfm. __________________ My Trucks: 1967 Chevrolet Short Wide Box 327 TH350 9" w/3.90 gears paint will be White - Current Project 1967 Chevrolet Custom LWB 283 TH400 3.73 Posi, no-AC, no-PS, no-PB, bench-seat, small-window - mostly orig driver 1967 Chevrolet CST LWB originally a 327 TH400 3.73 Posi AC PS PB, had Buddy Buckets, Small Window - parts truck 1967 Chevrolet CST LWB, 283 MT 3.73 had Buddy Buckets, Panoramic Window - parts truck 2001 Chevrolet 3500 2WD Crew Cab Dually 8.1L Allison White Last edited by 67 cst swb; 10-26-2007 at 01:25 AM.

10-25-2007, 04:55 PM	#22
JAKES 68GMC Aluminum slot wheels rule... Join Date: Jan 2006 Location: Maine Posts: 6,211	Re: C20 milage improvements I'd kill for 13mpg.

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